To assemble is a fag (4,2)
I believe the answer is:
roll up
I believe this is a double definition.
'to assemble' is the first definition.
'a fag' is the second definition.
(type of cigarette)
'is' acts as a link.
(Other definitions for roll up that I've seen before include "Arrive -- handmade cigarette (4,2;4-2)" , "smoke" , "Salesman's cry" , "Fairground barker's exhortation" , "Invitation to come" .)