Train app used regularly when heading west to find partner (4)
I believe the answer is:
pair
'partner' is the definition.
(both can mean to form a pairing)
'train app used regularly when heading west' is the wordplay.
'used regularly' indicates alternate letters.
'when heading west' is a reversal indicator (letters written right to left or east to west).
'train'+'app'='trainapp'
The alternating letters of 'trainapp' are 'riap'.
'riap' reversed gives 'PAIR'.
'to find' is the link.
(Other definitions for pair that I've seen before include "2" , "Couple - wed" , "Agreement to avoid division" , "Two of a similar type" , "Set of two" .)