Saint takes pen outside restaurant (6)
I believe the answer is:
bistro
'restaurant' is the definition.
(bistro is a kind of restaurant)
'saint takes pen outside' is the wordplay.
'saint takes' becomes 'st' (this might be a standard abbreviation which I don't know about).
'pen' becomes 'Biro' (Biro is a make of pen).
'outside' is an insertion indicator (some letters go outside others).
'st' put inside '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" .)