Fling? Witness pair inside (5)
I believe the answer is:
spree
'fling?' is the definition.
(I know that fling can be written as spree)
'witness pair inside' is the wordplay.
'witness' becomes 'see' (synonyms).
'pair' becomes 'pr' (abbreviation).
'inside' is an insertion indicator.
'see' placed around 'pr' is 'SPREE'.
(Other definitions for spree that I've seen before include "Session of over-indulgence" , "Unrestrained time" , "Carefree, lively outing" , "night on the razzle?" , "Beano" .)