Can seem a thousand short (3)
I believe the answer is:
loo
'can' is the definition.
(loo is a kind of can)
'seem a thousand short' is the wordplay.
'seem' becomes 'look' ('look' can be a synonym of 'seem').
'a thousand' becomes 'k' ('K' can be a synonym of 'thousand').
'short' suggests deleting specific letters (short can mean missing or absent).
'look' with 'k' removed is 'LOO'.
(Other definitions for loo that I've seen before include "A colloquial toilet" , "Privy" , "WC - old card game" , "Rest room" , "The little room - card game" .)