Pressure working on site with nitrogen (7)
I believe the answer is:
tension
'pressure' is the definition.
(thesaurus)
'working on site with nitrogen' is the wordplay.
'working' indicates an anagram.
'with' is a charade indicator (letters next to each other).
'nitrogen' becomes 'n' (N is the chemical symbol for nitrogen**).
'on'+'site'='onsite'
'onsite' with letters rearranged gives 'tensio'.
'tensio'+'n'='TENSION'
(Other definitions for tension that I've seen before include "The effect of force exerted in or on something" , "Mental or emotional strain" , "Assay" , "Anxiety" , "Intones (anag) - stress" .)