Stop, about to relax (5)
I believe the answer is:
cease
'stop' is the definition.
('cease' can be a synonym of 'stop')
'about to relax' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'to relax' becomes 'ease' (I've seen this before**).
'c'+'ease'='CEASE'
(Other definitions for cease that I've seen before include "Desist" , "Stop, finish" , "Political group" , "Discontinue" , "Culminate" .)