Cool quartet in past maybe concentrated (9)
I believe the answer is:
intensive
'concentrated' is the definition.
(synonyms)
'cool quartet in past' is the wordplay.
'cool' becomes 'in' ('in' can mean fashionable or cool).
'quartet' becomes 'IV' (Roman numerals for four).
'in' is an insertion indicator.
'past' becomes 'tense' (past tense).
'iv' put inside 'tense' is 'tensive'.
'in'+'tensive'='INTENSIVE'
'maybe' acts as a link.
(Other definitions for intensive that I've seen before include "Demanding" , "Concentrated" , "Very vigorous or thorough; concentrating" .)