Tramp's in contact with relation (7)
I believe the answer is:
'relation' is the definition.
(stepson is a kind of relation)
'tramp's in contact with' is the wordplay.
'tramp's' becomes 'step's' (to tramp is to step heavily).
'in contact with' becomes 'on' (I've seen this in other clues).
(Other definitions for stepson that I've seen before include "unrelated issue" , "No pests for male issue of spouse - but not of self" , "Subdues" , "Male child of spouse by earlier marriage" , "boy not sharing blood" .)