Assuming expert struggled tackling Mastermind's final (8)
I believe the answer is:
provided
'assuming' is the definition.
(provided certain assumed conditions are met)
'expert struggled tackling mastermind's final' is the wordplay.
'expert' becomes 'pro' (short for professional).
'struggled' becomes 'vied' (I've seen this in another clue).
'tackling' is an insertion indicator (to tackle is to take possession of).
'final' indicates one should take the final letters.
The final letter of 'mastermind' is 'd'.
'vied' placed around 'd' is 'vided'.
'pro'+'vided'='PROVIDED'
(Other definitions for provided that I've seen before include "If - supplied" , "furnished" , "Given" , "in the event" , "... on understanding that" .)