In short, 'ermine' is an expression (4)
I believe the answer is:
term
'an expression' is the definition.
(I've seen this before)
'in short ermine is' is the wordplay.
'in' says the answer is hidden in the clue.
'TERM' is hidden in the letters of 'short ermine is'.
(Other definitions for term that I've seen before include "end" , "Limited period; school session" , "Limited period of time" , "A fixed or limited time - or an expression" , "Part of the school year" .)