Be leading the deer that's slow (6)
I believe the answer is:
behind
'slow' is the definition.
(thesaurus)
'be leading the deer' is the wordplay.
'leading' is a charade indicator (letters next to each other) (some letters go in front of others).
'the deer' becomes 'hind' (hind is a kind of deer).
'be'+'hind'='BEHIND'
'that's' acts as a link.
(Other definitions for behind that I've seen before include "in van? No" , "Rear - losing" , "In arrears; guiding" , "support" , "Following - late" .)