Changes hat in place where just one at a time is allowed (9)
I believe the answer is:
turnstile
'place where just one at a time is allowed' is the definition.
(only one person can use a turnstile at once)
'changes hat' is the wordplay.
'changes' becomes 'turns' (turn is a kind of change).
'hat' becomes 'tile' (I've seen this before).
'turns'+'tile'='TURNSTILE'
'in' is the link.
(Other definitions for turnstile that I've seen before include "where spectators pass" , "Gate admitting one at a time" , "Rotating gate" , "Admission gate" , "Control for spectators' entry" .)