Acted up, finally, with respect to what a barworker did? (9)
I believe the answer is:
pretended
'acted' is the definition.
(pretending is a kind of acting)
'up finally with respect to what a barworker did?' is the wordplay.
'finally' suggests the final letters.
'with respect to' becomes 're' ('re' means regarding or about).
'what a barworker did?' becomes 'tended' (as in a bartender).
The final letter of 'up' is 'p'.
'p'+'re'+'tended'='PRETENDED'
(Other definitions for pretended that I've seen before include "Feigned, faked" , "Engaged in fantasy; simulated (an emotion)" , "Feigned, affected" , "Made believe with intent to deceive" , "Made believe, feigned," .)