Before Heather, Fred’s first affair (5)
I believe the answer is:
fling
'affair' is the definition.
(a fling is a romantic affair)
'before heather fred's first' is the wordplay.
'before' says to put letters next to each other.
'heather' becomes 'ling' (synonyms).
'fred's first' becomes 'f' (1st letter of 'fred').
'ling' put after 'f' is 'FLING'.
(Other definitions for fling that I've seen before include "Brief amour; throw" , "Brief affair; throw" , "Vigorous Scottish dance" , "Throw angrily in spell of indulgent pleasure" , "Brief relationship" .)