Freshen up when leaving hospital emergency room (5)
I believe the answer is:
renew
'freshen' is the definition.
(I know that freshen is a more specific form of the action renew)
'up when leaving hospital emergency room' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'leaving' indicates named letters should be taken away.
'hospital' becomes 'h' (abbreviation for hospital on maps).
'emergency room' becomes 'ER' (abbreviation popularized by US TV series).
'when' with 'h' taken away is 'wen'.
'wen'+'er'='wener'
'wener' written backwards gives 'RENEW'.
(Other definitions for renew that I've seen before include "Repeat for next year" , "Restore, resume" , "Make afresh" , "Take up again" , "Start afresh" .)