After stretch on couch, priest full of energy for flock (5,2,1,7)
I believe the answer is:
birds of a feather
'for flock' is the definition.
'after stretch on couch priest full' is the wordplay.
'after' says to put letters next to each other.
'stretch' becomes 'bird' (both can mean a period in jail).
'on' says to put letters next to each other.
'couch' becomes 'sofa' ('sofa' can be a synonym of 'couch').
'priest full' becomes 'feather' (I can't justify this - if you can you should believe this answer much more).
'sofa' next to 'feather' is 'sofafeather'.
'bird'+'sofafeather' is 'BIRDS OF A FEATHER'.
'of energy' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
(Other definitions for birds of a feather that I've seen before include "like-minded people" , "they combine well!" , "A kind" , "sitcom" , "Like people" .)