Precise pressure on lip (4)
I believe the answer is:
prim
'precise' is the definition.
(thesaurus)
'pressure on lip' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'on' means one lot of letters go next to another.
'lip' becomes 'rim' (synonyms).
'p'+'rim'='PRIM'
(Other definitions for prim that I've seen before include "Stiffly correct" , "Stiffly formal and proper" , "Prudish, proper" , "Schoolmarmish" , "Strait-laced - prudish" .)