Stretch out in park on course (7)
I believe the answer is:
Here is my best explanation:
'stretch out' is the definition.
'park on course' is the subsidiary indication.
'park' becomes 'rec' (I've seen this in other clues).
'on' says to put letters next to each other.
'course' becomes 'line' ('line' can be a synonym of 'course'**).
'in' is the link.
(Other definitions for recline that I've seen before include "Lounge", "sit back ...", "Lie back to rest", "Lie in a comfortable position", "Lie back and take it easy", "Be recumbent", "Lie back and relax", "Assume horizontal or leaning position as for rest", "Lean back, relax" and "Relax".)