Fellow travellers stretch over couch with something for a pillow (5,2,1,7)
I believe the answer is:
birds of a feather
'fellow travellers' is the definition.
(people with similar interests)
'stretch over couch with something for a pillow' is the wordplay.
'stretch' becomes 'bird' (both can mean a period in jail).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'couch' becomes 'sofa' (furniture item).
'with' means one lot of letters go next to another.
'something for a pillow' becomes 'feather' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'bird'+'sofa'+'feather'='BIRDS OF A FEATHER'
Can you help me to learn more?
(Other definitions for birds of a feather that I've seen before include "like-minded people" , "Likes" , "A kind" , "Like people" , "they combine well!" .)