Find fish round it under end of pier (6)
I believe the answer is:
rarity
'find' is the definition.
(thesaurus)
'fish round it under end of pier' is the wordplay.
'fish' becomes 'ray' (ray is a kind of fish**).
'round' indicates putting letters inside.
'under' says to put letters next to each other.
'end of' indicates one should take the final letters.
The last letter of 'pier' is 'r'.
'it' after 'r' is 'rit'.
'ray' placed around 'rit' is 'RARITY'.
(Other definitions for rarity that I've seen before include "It occurs infrequently" , "Curio (1)" , "Unlikely event" , "Unusual specimen" , "Curiosity, singularity" .)