One gate's damaged - but that was a long time ago (5,3)
I believe the answer is:
stone age
'long time ago' is the definition.
(I've seen this before)
'one gate's damaged' is the wordplay.
'damaged' indicates an anagram.
'one'+'gates'='onegates'
'onegates' anagrammed gives 'STONE-AGE'.
'but that was a' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for stone age that I've seen before include "Goat seen (anag.)" , "Earliest known period of human culture" , "Neolithic" , "before copper came in" , "Go, senate to ancient era before metal was discovered" .)