Best work by small boy, with little hesitation (7)
I believe the answer is:
optimum
'best' is the definition.
('optimum' can be similar in meaning to 'best')
'work by small boy with little hesitation' is the wordplay.
'work' becomes 'op' (abbreviation for opus**).
'by' says to put letters next to each other.
'small boy' becomes 'tim'.
'with' is a charade indicator (letters next to each other).
'little hesitation' becomes 'um'.
'op'+'tim'+'um'='OPTIMUM'
(Other definitions for optimum that I've seen before include "Most appropriate" , "The most favourable condition" , "Most favourable (conditions)" , "Best (result)" , "model" .)