True: sea can be cold (7)
I believe the answer is:
'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.
'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" .)