Secret vault mostly very cold (7)
I believe the answer is:
cryptic
'secret' is the definition.
(synonyms)
'vault mostly very cold' is the wordplay.
'vault' becomes 'crypt' (I've seen this before).
'mostly' means to remove the last letter (most but not all of the word).
'very cold' becomes 'icy' (I've seen this before).
'icy' with its final letter taken off is 'ic'.
'crypt'+'ic'='CRYPTIC'
(Other definitions for cryptic that I've seen before include "'Obscure, puzzling (7)'" , "(Of crossword) withindirect clues" , "Obscure or puzzling (as our page neighbour Crosaire)" , "Like our other puzzle" , "Concealing meaning in puzzling way as Crosaire does" .)