Deal set out about marathon, possibly, in Sussex town? (7)
I believe the answer is:
arundel
'sussex town?' is the definition.
(I've seen this before)
'deal set out about marathon possibly' is the wordplay.
'set out' is an anagram indicator.
'about' is an insertion indicator.
'marathon possibly' becomes 'run' (marathon is a kind of run).
'deal' with letters rearranged gives 'adel'.
'adel' placed around 'run' is 'ARUNDEL'.
'in' acts as a link.
(Other definitions for arundel that I've seen before include "Norfolk seat" , "West Sussex town with a castle" , "Castle in W. Sussex" , "English public school" , "somewhere in W Sussex" .)