Annoying nitwit coming between Henry and Heather (8)
I believe the answer is:
hassling
'annoying' is the definition.
(hassling is a kind of annoying)
'nitwit coming between henry and heather' is the wordplay.
'nitwit' becomes 'ass' (I've seen this in another clue).
'coming between' means one lot of letters goes inside another.
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'and' is a charade indicator (letters next to each other).
'heather' becomes 'ling' (synonyms).
'h'+'ling'='hling'
'ass' going into 'hling' is 'HASSLING'.
(Another definition for hassling that I've seen is " Annoying continually".)