Stray that's pale turning red (6)
I believe the answer is:
wander
'stray' is the definition.
(I know that stray can be written as wander)
'pale turning red' is the wordplay.
'pale' becomes 'wan' (wan can mean pale or grey).
'turning' is a reversal indicator.
'red' back-to-front is 'der'.
'wan'+'der'='WANDER'
'that's' is the link.
(Other definitions for wander that I've seen before include "Go to and fro" , "endless travel" , "Move about aimlessly" , "Roam around aimlessly" , "Stroll" .)