Thoroughgoing quartet entering in future, perhaps (9)
I believe the answer is:
intensive
'thoroughgoing' is the definition.
(thesaurus)
'quartet entering in future perhaps' is the wordplay.
'quartet' becomes 'IV' (Roman numerals for four).
'entering' means one lot of letters goes inside another.
'future perhaps' becomes 'tense' (future tense).
'in'+'tense'='intense'
'iv' placed inside 'intense' is 'INTENSIVE'.
(Other definitions for intensive that I've seen before include "Concentrated" , "Very vigorous or thorough; concentrating" , "Demanding" .)