Egg on single girl to be spendthrift (8)
I believe the answer is:
prodigal
'be spendthrift' is the definition.
('prodigal' can be similar in meaning to 'spendthrift')
'egg on single girl' is the wordplay.
'egg on' becomes 'prod' ('prod' can be a synonym of 'egg on').
'single' becomes 'i' (one).
'girl' becomes 'gal' ('gal' can be a synonym of 'girl').
'prod'+'i'+'gal'='PRODIGAL'
'to' is the link.
(Other definitions for prodigal that I've seen before include "Wasteful - gold pair (anag)" , "Recklessly wasteful or extravagant" , "Grip load (anag.)" , "reckless" , "This son got the fatted calf" .)