A cold fish admitting Romeo's tender (6)
I believe the answer is:
accord
'tender' is the definition.
(thesaurus)
'a cold fish admitting romeo's' is the wordplay.
'a cold' becomes 'ac' (I've seen this in other clues**).
'fish' becomes 'cod' (cod is a kind of fish).
'admitting' is an insertion indicator.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'cod' going around 'r' is 'cord'.
'ac'+'cord'='ACCORD'
(Other definitions for accord that I've seen before include "Harmonize" , "'Agreement, treaty (6)'" , "Official agreement; harmony" , "Match" , "concert" .)