No good piercing a fish when doing this! (7)
I believe the answer is:
angling
'no good piercing a fish when doing this' is the definition.
The definition suggests a singular noun which matches the answer. I also know that 'fish' relates to this answer.
'no good piercing a fish' is the wordplay.
'no good' becomes 'ng' (abbreviation).
'piercing' means one lot of letters goes inside another (some letters pierce a hole in a word).
'fish' becomes 'ling' (ling is a kind of fish).
'a'+'ling'='aling'
'ng' going into 'aling' is 'ANGLING'.
(Other definitions for angling that I've seen before include "Indirectly seeking" , "Pointing" , "Fishing with hook and line" , "Walton practised it" , "Fishing with rod and line" .)