Street philosopher one unlikely to give up (6)
I believe the answer is:
stayer
'one unlikely to give up' is the definition.
(I've seen this before)
'street philosopher' is the wordplay.
'street' becomes 'st' (abbreviation).
'philosopher' becomes 'ayer' (AJ Ayer).
'st'+'ayer'='STAYER'
(Other definitions for stayer that I've seen before include "tenacious sort" , "one unlikely to give up" , "One that can last the course" , "One with lasting qualities" , "Horse with stamina" .)