Me, I will participate in most of opportunity to get fitter? (8)
I believe the answer is:
mechanic
'fitter?' is the definition.
(car fitter)
'me i will participate in most of opportunity' is the wordplay.
'will participate in' indicates putting letters inside.
'most of' means to remove the last letter.
'opportunity' becomes 'chance' (synonyms).
'chance' with its final letter taken off is 'chanc'.
'i' placed inside 'chanc' is 'chanic'.
'me'+'chanic'='MECHANIC'
'to get' acts as a link.
(Other definitions for mechanic that I've seen before include "Mender of machines" , "Garage worker" , "Craftsman skilled in machines" , "One repairs cars, say" , "Person who repairs and maintains cars, say" .)