Period under psychotherapist is withering experience (9)
I believe the answer is:
shrinkage
'withering experience' is the definition.
'period under psychotherapist' is the wordplay.
'period' becomes 'age' (age is a kind of period).
'under' says to put letters next to each other (in a down clue, some letters go below others).
'psychotherapist' becomes 'shrink' (I can't justify this - if you can you should give a lot more credence to this answer).
'age' after 'shrink' is 'SHRINKAGE'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for shrinkage that I've seen before include "getting smaller" , "Reduction in volume" , "Process of shrivelling or contracting" , "Gin shaker? (anag)" , "lessening" .)