Friend outside trendy theatre, finally, for afternoon show (7)
I believe the answer is:
matinee
'afternoon show' is the definition.
(I've seen this before)
'friend outside trendy theatre finally' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend).
'outside' indicates putting letters inside.
'trendy' becomes 'in'.
'finally' indicates one should take the final letters.
The final letter of 'theatre' is 'e'.
'mate' going around 'in' is 'matine'.
'matine'+'e'='MATINEE'
'for' acts as a link.
(Other definitions for matinee that I've seen before include "Afternoon show" , "After lunch, show" , "Afternoon showing of play" , "Play, perhaps" , "Daytime performance of play etc" .)