True: sea can be cold (7)
I believe the answer is:
austere
'cold' is the definition.
(I know that cold can be written as austere)
'true sea can be' is the wordplay.
'can be' indicates anagramming the letters.
'true'+'sea'='truesea'
'truesea' anagrammed gives 'AUSTERE'.
(Other definitions for austere that I've seen before include "Abstemious and grave" , "'Spartan, simple (7)'" , "Severe or strict in attitude" , "Stern, grave" , "Sure eat up and become without luxury" .)