Force per unit area (8)
I believe the answer is:
pressure
'force per unit area' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for pressure that I've seen before include "Force applied to a surface" , "Type of organised group which tries to influence politicians" , "Coercion, intimidation" , "Force per unit area" , "Coercion or forceful persuasion" .)