This paper deals with existence, uniqueness and global behaviour of a positive solution for the fractional boundary value problem D β (ψ(x) p (D α u)) = a(x)u σ in (0, 1) with the condition lim x→0 D β−1 (ψ(x) p (D α u(x))) = lim x→1 ψ(x) p (D α u(x)) = 0 and lim x→0 D α−1 u(x) = u(1) = 0, where β, α ∈ (1, 2], p (t) = t|t| p−2 , p > 1, σ ∈ (1 − p, p − 1), the differential operator is taken in the Riemann-Liouville sense and ψ, a : (0, 1) −→ R are non-negative and continuous functions that may are singular at x = 0 or x = 1 and satisfies some appropriate conditions.