Nothing fine about grand affair (5)
I believe the answer is:
fling
'affair' is the definition.
(a fling is a romantic affair)
'nothing fine about grand' is the wordplay.
'nothing' becomes 'nil' (eg nil is zero in football).
'fine' becomes 'f' (abbreviation e.g. condition of old coins, books).
'about' says the letters should be written in reverse.
'grand' becomes 'g' (G is short for grand as in a thousand).
'nil'+'f'='nilf'
'nilf' written backwards gives 'flin'.
'flin'+'g'='FLING'
(Other definitions for fling that I've seen before include "Brief affair - chuck" , "See 7" , "good time" , "Launch or propel" , "Hurl" .)