Single decoration, apt reward when viewed in retrospect (5)
I believe the answer is:
motif
'single decoration' is the definition.
(a motif is a repeated design)
'apt reward when viewed in retrospect' is the wordplay.
'apt' becomes 'fit' (I've seen this in another clue).
'reward' becomes 'OM' (Order of Merit award).
'when viewed in retrospect' shows that the letters should be reversed in order.
'fit'+'om'='fitom'
'fitom' back-to-front is 'MOTIF'.
(Other definitions for motif that I've seen before include "symbolic ornament" , "Recurrent theme in music..." , "design of some significance" , "Recurring theme in musical work" , "Characteristic theme" .)