Bit of a gadabout, but expert (3)
I believe the answer is:
dab
'expert' is the definition.
(I've seen this before)
'bit of a gadabout but' is the wordplay.
'bit of' says the answer is hidden in the clue.
'DAB' is hidden in the letters of 'gadabout but'.
(Other definitions for dab that I've seen before include "Gently wipe" , "Light stroke" , "Bad sort of small fish" , "A bad sort of little fish" , "Expert; flatfish" .)