Visit work building firstly in very filthy place (4,2)
I believe the answer is:
stop by
'visit' is the definition.
(thesaurus)
'work building firstly in very filthy place' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'firstly' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'very filthy place' becomes 'sty'.
The first letter of 'building' is 'b'.
'op'+'b'='opb'
'opb' put into 'sty' is 'STOP BY'.
(Other definitions for stop by that I've seen before include "Drop in" , "Call as a visitor" , "Pop in" .)