V fishy stain under unopened receptacle (7)
I believe the answer is:
against
'v' is the definition.
(v means versus in sports fixtures)
'fishy stain under unopened receptacle' is the wordplay.
'fishy' indicates anagramming the letters.
'under' says to put letters next to each other.
'unopened' means to remove the first letter.
'receptacle' becomes 'bag'.
'bag' with its first letter removed is 'ag'.
'stain' anagrammed gives 'ainst'.
'ainst' after 'ag' is 'AGAINST'.
(Other definitions for against that I've seen before include "on the other side" , "Anti, touching" , "In opposition to" , "Opposed" , "Facing" .)