President with certain force (8)
I believe the answer is:
pressure
'force' is the definition.
(I know that force can be written as pressure)
'president with certain' is the wordplay.
'president with' becomes 'pres' (I can't justify this - if you can you should believe this answer much more).
'certain' becomes 'sure' (synonyms).
'pres'+'sure'='PRESSURE'
Can you help me to learn more?
(Other definitions for pressure that I've seen before include "Force measured in pascals" , "Coercion or forceful persuasion" , "tension" , "Coercion, intimidation" , "Force per unit area" .)