Extremely earnest in past, perhaps (7)
I believe the answer is:
intense
'extremely earnest' is the definition.
(I've seen this before)
'in past perhaps' is the wordplay.
'past perhaps' becomes 'tense' (grammar**).
'in'+'tense'='INTENSE'
(Other definitions for intense that I've seen before include "Set nine (anag.)" , "Concentrated, deeply felt" , "Deeply felt or vehement" , "Strong in quality or degree - perhaps a feeling" , "Great" .)