New hat style could be secret (8)
I believe the answer is:
stealthy
'be secret' is the definition.
(I've seen this before)
'new hat style' is the wordplay.
'new' indicates an anagram.
'hat'+'style'='hatstyle'
'hatstyle' is an anagram of 'STEALTHY'.
'could' is the link.
(Other definitions for stealthy that I've seen before include "'Furtive, surreptitious (8)'" , "Acting in quiet or secret way" , "Secretive, silent" , "Moving surreptitiously" , "Acting in furtive or covert manner" .)