Demand payment for some of the complex activities (5)
I believe the answer is:
exact
'demand payment' is the definition.
(as in exacting a tax)
'some of the complex activities' is the wordplay.
'some of' indicates the answer is hidden within the clue.
'EXACT' is hidden in the letters of 'complex activities'.
'for' is the link.
(Other definitions for exact that I've seen before include "accurate in all details" , "Demand and obtain" , "On the nail" , "19 [WREST]" , "without rounding" .)