Old boy mostly existing with love in limbo (8)
I believe the answer is:
oblivion
'limbo' is the definition.
(synonyms)
'old boy mostly existing with love in' is the wordplay.
'old boy' becomes 'ob' (abbreviation).
'mostly' means to remove the last letter.
'existing' becomes 'living' ('live' can be a synonym of 'exist').
'with' indicates putting letters inside.
'love in' becomes 'o' (In tennis, 'love' means 'zero'. I am not sure about the 'in' bit.).
'living' with its last letter taken away is 'livin'.
'livin' enclosing 'o' is 'livion'.
'ob'+'livion'='OBLIVION'
(Other definitions for oblivion that I've seen before include "Total extinction" , "State of being totally unconscious" , "State of being totally forgotten" , "The state of being unconscious of what is happening" , "Obscurity" .)