Counter on small pool (7)
I believe the answer is:
respond
'counter' is the definition.
(I've seen this in another clue)
'on small pool' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'small' becomes 's' (abbreviation - e.g. clothes size).
'pool' becomes 'pond' (synonyms).
're'+'s'+'pond'='RESPOND'
(Other definitions for respond that I've seen before include "Reply - react" , "Answer back" , "Answer or react" , "State in reply" , "Come back" .)