Gym relies on developing coastal town (4,5)
I believe the answer is:
lyme regis
'coastal town' is the definition.
(town in Dorset)
'gym relies on developing' is the wordplay.
'on' means one lot of letters go next to another.
'developing' indicates anagramming the letters.
'gym'+'relies'='gymrelies'
'gymrelies' is an anagram of 'LYME REGIS'.
(Other definitions for lyme regis that I've seen before include "Coastal town of Dorset" , "French Lieut.'s Woman town (Fowles)" , "Dorset coastal resort" , "first section (of piece of music)" .)