Underscore “in years” (7)
I believe the answer is:
teenage
'underscore' is the definition.
The answer and definition are not the same part of speech.
'in years' is the wordplay.
'in' becomes 'teen' (I can't explain this - if you can you should give a lot more credence to this answer).
'years' becomes 'age' ('age' can be a synonym of 'years').
'teen'+'age'='TEENAGE'
Can you help me to learn more?
(Other definitions for teenage that I've seen before include "Youthful" , "Like a category of youngsters" , "Like one thirteen-nineteen years old" , "Relatively young" , "Eat gene to become young" .)