Parasites initially horrify and anger the boy (7-2)
I believe the answer is:
hangers-on
'parasites' is the definition.
(hanger-on is a kind of parasite)
'initially horrify and anger the boy' is the wordplay.
'initially' suggests taking the first letters.
'and' says to put letters next to each other.
'the boy' becomes 'son' (synonyms).
The initial letter of 'horrify' is 'h'.
'h'+'anger'+'son'='HANGERS-ON'
(Other definitions for hangers-on that I've seen before include "Sycophantic associates" , "Persistent (and annoying) companions" , "Minions" , "Sycophants, unwanted followers" , "Sycophantic followers - dependants" .)