Move slowly as monster that's lost leg (4)
'move slowly' is the definition.
'monster that's lost leg' is the wordplay.
'monster' becomes 'dragon' (dragon is a kind of monster).
'that's lost' suggests deleting specific letters.
'leg' becomes 'on' (cricket term, fielding positions).
'dragon' with 'on' taken out is 'DRAG'.
'as' acts as a link.
(Other definitions for drag that I've seen before include "Pull - dredge - transvestite habit", "Pantomime dame's get-up", "Haul - queen's garb?", "such a nuisance", "Pull along; pass slowly".)