Saint takes pen outside restaurant (6)
I believe the answer is:
bistro
'restaurant' is the definition.
(I know that bistro is a type of restaurant)
'saint takes pen outside' is the wordplay.
'saint takes' becomes 'st' (this could be a standard abbreviation which I don't know about).
'pen' becomes 'Biro' (Biro is a make of pen).
'outside' means one lot of letters goes inside another.
'st' put into 'biro' is 'BISTRO'.
Can you help me to learn more?
(Other definitions for bistro that I've seen before include "Small bar-cum-restaurant" , "Small French bar and restaurant" , "Cafe bar" , "Eating place" , "Cafe/restaurant" .)