Happen to upset terrapins (9)
I believe the answer is:
transpire
'happen' is the definition.
(I know this)
'upset terrapins' is the wordplay.
'upset' indicates anagramming the letters.
'terrapins' is an anagram of 'TRANSPIRE'.
'to' acts as a link.
(Other definitions for transpire that I've seen before include "Lose water vapour" , "exhale" , "come to light" , "revealed" , "Happen[s]" .)