Most stretched out legs, not painfully (7)
I believe the answer is:
longest
'most stretched out' is the definition.
(long can mean stretched out)
'legs not painfully' is the wordplay.
'painfully' indicates an anagram (letters get injured).
'legs'+'not'='legsnot'
'legsnot' with letters rearranged gives 'LONGEST'.
(Other definitions for longest that I've seen before include "Extending farthest" , "For the most time" , "Least short" , "Of greatest duration" , "Having the most length" .)