Mule's small limb (foot missing) (7)
I believe the answer is:
slipper
'mule's' is the definition.
(I've seen this in another clue)
'small limb foot missing' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'limb' becomes 'flipper' (flat limb used for swimming, seals etc.).
'foot' becomes 'f' (abbreviation).
'missing' suggests deleting specific letters.
'flipper' with 'f' taken away is 'lipper'.
's'+'lipper'='SLIPPER'
(Other definitions for slipper that I've seen before include "House shoe" , "Type of footwear" , "Comfortable footwear" , "One losing footing" , "Ripples (anag.)" .)