A series on hospital department will be caustic (10)
I believe the answer is:
astringent
'caustic' is the definition.
(thesaurus)
'a series on hospital department' is the wordplay.
'series' becomes 'string' (sequence of similar items or events).
'on' means one lot of letters go next to another.
'hospital department' becomes 'ent'.
'a'+'string'+'ent'='ASTRINGENT'
'will be' acts as a link.
(Other definitions for astringent that I've seen before include "(Medication) causing contraction - stern giant (anag)" , "Lotion" , "Scathing" , "description of wine?" , "Severe - checking the flow of blood" .)