Stocky type to finish concert up in old hotel (9)
I believe the answer is:
endomorph
'stocky type' is the definition.
(someone with a stocky body)
'concert up in old hotel' is the wordplay.
'concert' becomes 'prom' (abbreviation for promenade concert).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'in' is an insertion indicator.
'old' becomes 'endo' (I can't justify this - if you can you should believe this answer much more).
'hotel' becomes 'h' (phonetic alphabet).
'prom' reversed gives 'morp'.
'endo'+'h'='endoh'
'morp' inserted inside 'endoh' is 'ENDOMORPH'.
'to finish' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
(Other definitions for endomorph that I've seen before include "fattish figure" , "Portly sort" , "Stocky person" , "Fatty type" , "Heavy person" .)