It's great if you're old enough (3)
I believe the answer is:
age
'old enough' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'it's great if you're' is the wordplay.
I cannot quite see how this works, but
'great' could be 'g' (abbreviation in place names) and 'g' is found in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is located in the answer.
A single letter 'a' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for age that I've seen before include "Years lived so far" , "Turkish commander" , "16/18 [INDUSTRIAL REVOLUTION]" , "Get old" , "Very long time" .)