Stop a trim teen getting out of shape (9)
I believe the answer is:
terminate
'stop' is the definition.
('terminate' can be a synonym of 'stop')
'a trim teen getting out of shape' is the wordplay.
'getting' says to put letters next to each other.
'out of shape' indicates anagramming the letters.
'a'+'trim'+'teen'='atrimteen'
'atrimteen' is an anagram of 'TERMINATE'.
(Other definitions for terminate that I've seen before include "Put an end to, discontinue" , "Stop" , "Put an end to, finish" , "Bring to an end" , "Finalise" .)