Factory dropping line is blow (4)
I believe the answer is:
pant
'is blow' is the definition.
(thesaurus)
'factory dropping line' is the wordplay.
'factory' becomes 'plant' (I've seen this before).
'dropping' suggests deleting specific letters.
'line' becomes 'l'.
'plant' with 'l' removed is 'PANT'.
(Other definitions for pant that I've seen before include "Sound out of breath" , "Struggle for breath" , "Breathe hard" , "Breathe noisily after running" , "Breathe heavily" .)