Fashionable decades provided unknown boost (9)
I believe the answer is:
intensify
'boost' is the definition.
'fashionable decades provided unknown' is the wordplay.
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'decades' becomes 'tens' ('ten' can be a synonym of 'decade').
'provided' becomes 'if'.
'unknown' becomes 'y' (algebra).
'in'+'tens'+'if'+'y'='INTENSIFY'
(Other definitions for intensify that I've seen before include "Fan" , "Make stronger" .)