Extremely tricky exercise for model (4)
I believe the answer is:
type
'model' is the definition.
(thesaurus)
'extremely tricky exercise' is the wordplay.
'extremely' means to remove the middle letters.
'exercise' becomes 'pe' (Physical Education).
'tricky' with its centre removed is 'ty'.
'ty'+'pe'='TYPE'
'for' is the link.
(Other definitions for type that I've seen before include "Category; use keyboard" , "get words on to paper?" , "Use keys" , "of a certain kind" , "Write letters" .)