Stretch one leg, at random (8)
I believe the answer is:
elongate
'stretch' is the definition.
('elongate' can be a synonym of 'stretch')
'one leg at random' is the wordplay.
'random' indicates an anagram.
'one'+'leg'+'at'='onelegat'
'onelegat' with letters rearranged gives 'ELONGATE'.
(Other definitions for elongate that I've seen before include "Lengthen or prolong" , "Open out" , "Stretch lengthways" , "very thin" , "Draw out" .)