A psychiatrist has to become smaller (6)
I believe the answer is:
shrink
I believe this clue is a double definition.
'a psychiatrist' is the first definition.
(informal term for a psychiatrist)
'become smaller' is the second definition.
(I've seen this before)
'has to' is the link.
(Other definitions for shrink that I've seen before include "Grow smaller like slang psychiatrist" , "Decrease - therapist" , "Withdraw from" , "Reduce in size" , "decline" .)