More appropriate for assistant, retired, to return (5)
I believe the answer is:
apter
'more appropriate' is the definition.
('apt' can be a synonym of 'appropriate')
'for assistant retired to return' is the wordplay.
'for assistant' becomes 'ap' (I can't explain this - if you can you should believe this answer much more).
'retired' becomes 'ret' (e.g. following the name of a retired army officer).
'to return' says the letters should be written in reverse.
'ret' written backwards gives 'ter'.
'ap'+'ter'='APTER'
Can you help me to learn more?
(Other definitions for apter that I've seen before include "More suitable - prate (anag)" , "Fitter" .)