Dim-witted Henry bitten by bloodsucker (5)
I believe the answer is:
thick
'dim-witted' is the definition.
(thesaurus)
'henry bitten by bloodsucker' is the wordplay.
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'bitten by' is an insertion indicator (I've seen this in other clues).
'bloodsucker' becomes 'tick' (blood-sucking parasite).
'h' put inside 'tick' is 'THICK'.
(Other definitions for thick that I've seen before include "Crowded" , "Stupid (as two short planks?)" , "having such a head?" , "Wide, broad" , "Stupid; (of fog) dense" .)