Spur putting pressure on staff (4)
I believe the answer is:
prod
'spur' is the definition.
(synonyms)
'pressure on staff' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'staff' becomes 'rod' (both can mean a pole or stick).
'p'+'rod'='PROD'
'putting' is the link.
(Other definitions for prod that I've seen before include "Poke, stimulate into action" , "Poke with stiff finger to urge" , "Dig" , "Poke or spur" , "Induce action (with stick etc?)" .)