Very old eat in canteen, just outside, strangely (7)
I believe the answer is:
ancient
'very old' is the definition.
(I've seen this before)
'eat in canteen just outside strangely' is the wordplay.
'just outside' suggests removing the centre (just the outside letters of the word).
'strangely' is an anagram indicator.
'canteen' with its middle removed is 'cn'.
'eat'+'in'+'cn'='eatincn'
'eatincn' is an anagram of 'ANCIENT'.
(Other definitions for ancient that I've seen before include "Of times long past" , "From the distant past" , "Very old, like some civilizations" , "showing signs of wear?" , "Antiquated" .)