Full theatre allowed start of expansion (7)
I believe the answer is:
replete
'full' is the definition.
('replete' can be a synonym of 'full')
'theatre allowed start of expansion' is the wordplay.
'theatre' becomes 'rep' (I've seen this in other clues).
'allowed' becomes 'let' (I've seen this before).
'start of' suggests taking the first letters.
The first letter of 'expansion' is 'e'.
'rep'+'let'+'e'='REPLETE'
(Other definitions for replete that I've seen before include "Full of food, gorged" , "Full, satiated" , "Filled to satisfaction" , "Fully satisfied" , "unable to take any more" .)