One trust supporting hospital department in full (8)
I believe the answer is:
entirely
'in full' is the definition.
'one trust supporting hospital department' is the wordplay.
'one' becomes 'I' (Roman numeral).
'trust' becomes 'rely' (I've seen this in other clues).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'hospital department' becomes 'ENT' (abbreviation for Ear, Nose and Throat department).
'i'+'rely'='irely'
'irely' put after 'ent' is 'ENTIRELY'.
(Other definitions for entirely that I've seen before include "Without reservation" , "Altogether" , "Wholly, completly" , "without qualification _" , "'Fully, wholly (8)'" .)