Constructional phrasemes are sometimes difficult to distinguish from 'complex collocations' or phraseological imbrications, which in turn could overlap with verbal and adverbial idioms, depending on whether their verbal element is included within the phraseme. The most prototypical cases are those in which an adverbial idiom frequently co-occurs with a verb, thus forming a collocation by imbrication between both parts. This verb may in turn present a striking lexical and syntactic variation by nominalisation. Therefore, these constructions do not show a uniform or predictable behaviour, and it is convenient to examine them on a case-by-case basis, detailing their combinatorial restrictions. Within this line, the present work studies -using qualitative and quantitative criteria -the external combinatorics of the Spanish adverbial idiom a pedir de boca within a large electronic textual corpus (esTenTen18). The sequence appears on 3330 occasions in its continuous form (0.17 per million words), plus 175 in discontinuous form. The verb salir predominates (1706 co-occurrences), but there are also 71 other verbs which combine on 398 occasions with a pedir de boca and which were grouped according to their semantic class. This results in four groups of verbs that are synonymic or hyponymic with respect to their prototype. Given that the basis of this complex collocation turns out to be an entire semantic class, and that each of its verbs has a highly variable frequency, it can be considered a constructional phraseme, or phraseological construction, whose fixedness is more conceptual than lexical-grammatical.