Do you ultimately collaborate in fact? (8)
I believe the answer is:
actually
'in fact?' is the definition.
('actually' can be a synonym of 'in fact')
'do you ultimately collaborate' is the wordplay.
'do' becomes 'act' (I've seen this before).
'ultimately' suggests the final letters.
'collaborate' becomes 'ally' (ally can mean to collaborate or work with someone).
The final letter of 'you' is 'u'.
'act'+'u'+'ally'='ACTUALLY'
(Other definitions for actually that I've seen before include "As a matter of fact" , "Really existing" , "A lay cult may be existing in fact" , "In reality" , "Really, in fact" .)