Ruffian in hospital in tower (4)
I believe the answer is:
thug
'ruffian' is the definition.
(thesaurus)
'hospital in tower' is the wordplay.
'hospital' becomes 'h'.
'in' means one lot of letters goes inside another.
'tower' becomes 'tug' (something that tows).
'h' put into 'tug' is 'THUG'.
'in' is the link.
(Other definitions for thug that I've seen before include "Violent ruffian" , "violent man" , "undesirable character" , "Rough" , "Burrowing crustacean" .)