Pressure on worker to gasp (4)
I believe the answer is:
pant
'to gasp' is the definition.
('pant' can be a synonym of 'gasp')
'pressure on worker' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'worker' becomes 'ant' (worker is a type of ant).
'p'+'ant'='PANT'
(Other definitions for pant that I've seen before include "Breathe hard" , "Breathe noisily after running" , "Breath rapidly" , "Struggle for breath" , "Sound out of breath" .)