Doing nothing to identify fish (6)
I believe the answer is:
idling
'doing nothing' is the definition.
(to idle is to do nothing)
'identify fish' is the wordplay.
'identify' becomes 'id' (I can't justify this - if you can you should believe this answer much more).
'fish' becomes 'ling' (ling is a kind of fish).
'id'+'ling'='IDLING'
'to' is the link.
Can you help me to learn more?
(Other definitions for idling that I've seen before include "Loafing or just ticking over" , "'Lazing, loafing (6)'" , "Being indolent" , "Being unoccupied" , "Wasting time" .)