Sort of card, thin but not large (3)
I believe the answer is:
sim
'sort of card' is the definition.
(as in a SIM card)
'thin but not large' is the wordplay.
'thin' becomes 'slim' (similar in meaning).
'but not' indicates named letters should be taken away.
'large' becomes 'L' (eg in clothes sizes).
'slim' with 'l' taken away is 'SIM'.
(Other definitions for sim that I've seen before include "Role-playing game" , "Identifying card in a mobile phone" , "computer game" , "Information carrier" , "Card storing data" .)