Unwell young mare loses coat (3)
I believe the answer is:
ill
'unwell' is the definition.
(similar in meaning)
'young mare loses coat' is the wordplay.
'young mare' becomes 'filly' (young female horse).
'loses coat' says to take the centre (letters which coat the word are removed).
The middle letters of 'filly' are 'ILL'.
(Other definitions for ill that I've seen before include "US state" , "Groggy" , "Not well, sick" , "Unwell, sick" , "Off colour?" .)