Crossword Genius

Extremely earnest in past, perhaps (7)

Ross

I believe the answer is:

intense

Here is my best explanation:

'extremely earnest' is the definition.
(I've seen this before)

'in past perhaps' is the wordplay.
'past perhaps' becomes 'tense' (grammar**).
'in'+'tense'='INTENSE'

This clue was last seen in The Telegraph