Space to stretch, sleep regularly, and prepare (7)
I believe the answer is:
legroom
'space to stretch' is the definition.
(I've seen this before)
'sleep regularly and prepare' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'and' is a charade indicator (letters next to each other).
'prepare' becomes 'groom' (synonyms).
The alternating letters of 'sleep' are 'le'.
'le'+'groom'='LEGROOM'
(Other definitions for legroom that I've seen before include "Space to stretch for seated person" , "Space between seat rows" , "Space often lacking on a plane!" , "Space needed to avoid cramped position" , "Space in front of a seat" .)