Picture the writer’s opposed to, enormously at first (7)
I believe the answer is:
imagine
'picture' is the definition.
(picture can mean to visualise or imagine)
'writer's opposed to enormously at first' is the wordplay.
'writer's' becomes 'I'm' ('I am' referring to the writer of the clue).
'opposed to' becomes 'agin' (I've seen this in other clues).
'at first' says to take the initial letters.
The initial letter of 'enormously' is 'e'.
'im'+'agin'+'e'='IMAGINE'
'the' acts as a link.
(Other definitions for imagine that I've seen before include "Conjure up in mind, visualise" , "Suspect" , "that Lennon composed" , "A Gemini? (anag)" , "Conjure up in mind,fancy" .)