On reflection, reveal film in short letter (7)
I believe the answer is:
notelet
'short letter' is the definition.
(I've seen this before)
'on reflection reveal film' is the wordplay.
'on reflection' shows that the letters should be reversed in order.
'reveal' becomes 'leton' ('let on' can be a synonym of 'reveal').
'film' becomes 'et'.
'leton' in reverse letter order is 'notel'.
'notel'+'et'='NOTELET'
'in' acts as a link.
(Other definitions for notelet that I've seen before include "means of communication" , "Paper folded" , "correspondence" , "short written message" , "Brief communication" .)