Expert angler keeps twirling fly (4)
I believe the answer is:
gnat
'fly' is the definition.
(type of insect)
'expert angler keeps twirling' is the wordplay.
'keeps' indicates the answer is hidden within the clue.
'twirling' says the letters should be written in reverse.
'tang' can be found hidden inside 'expert angler'.
'tang' in reverse letter order is 'GNAT'.
(Other definitions for gnat that I've seen before include "Small (swarming) fly" , "Small biting fly" , "Biting pest" , "annoying flier" , "little irritation" .)