Frequently of tender years (5)
I believe the answer is:
often
'frequently' is the definition.
('often' can be a synonym of 'frequently')
'of tender years' is the wordplay.
'tender years' becomes 'ten' (I can't justify this - if you can you should give a lot more credence to this answer).
'of'+'ten'='OFTEN'
Can you help me to learn more?
(Other definitions for often that I've seen before include "Frequently, many times" , "Perhaps daily" , "on numerous occasions" , "Frequently, not seldom" , "Repeatedly, many times" .)