Hospital department supporting line that's tough (9)
I believe the answer is:
stringent
'tough' is the definition.
(I've seen this before)
'hospital department supporting line' is the wordplay.
'hospital department' becomes 'ENT' (Ear, Nose and Throat department).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'line' becomes 'string' (string is a kind of line).
'ent' after 'string' is 'STRINGENT'.
'that's' is the link.
(Other definitions for stringent that I've seen before include "firm" , "Tough" , "Strict, rigorous" , "binding" , "Requiring strict adherence to detail" .)