Clumsy fiancé regularly falls out with personal trainer (5)
I believe the answer is:
inept
'clumsy' is the definition.
(synonyms)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for inept that I've seen before include "Clumsy - silly" , "Amateurish" , "Incompetent or gauche" , "Maladroit" , "Not competent" .)