Special lifts to be installed in hospital recess (4)
I believe the answer is:
apse
'recess' is the definition.
(apse is a kind of recess)
'special lifts to be installed in hospital' is the wordplay.
'special' becomes 'sp' (abbreviation).
'lifts' says the letters should be written in reverse (I've seen 'lifting' mean this).
'to be installed in' means one lot of letters goes inside another.
'hospital' becomes 'ae' (I am not sure about this - if you are sure you should believe this answer much more).
'sp' back-to-front is 'ps'.
'ps' put into 'ae' is 'APSE'.
Can you help me to learn more?
(Other definitions for apse that I've seen before include "Church recess with domed or arched roof" , "Semicircular end of a church, behind the altar" , "Projection to the east end of a church" , "room for an altar" , "space for altar" .)