Tea's gone off – 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 in another clue)
'tea's gone off' is the wordplay.
'off' is an anagram indicator.
'teas'+'gone'='teasgone'
'teasgone' with letters rearranged gives 'STONE-AGE'.
'aeur but that was a' acts as a 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 "Pre-historic era" , "Earliest known period of human culture" , "time long ago" , "before copper came in" , "Hard time" .)