Join one heading for Exeter (5)
I believe the answer is:
unite
'join' is the definition.
(I know that join can be written as unite)
'one heading for exeter' is the wordplay.
'one' becomes 'unit' (synonyms).
'heading for' says to take the initial letters.
The initial letter of 'exeter' is 'e'.
'unit'+'e'='UNITE'
(Other definitions for unite that I've seen before include "Get together" , "Wed?" , "Become an integrated whole" , "Gather together as one" , "pool resources" .)