Parasite ran back to change sides with Banquo's son (7)

'banquo's son' is the definition.
(I've seen this in another clue)

'parasite ran back to change sides' is the wordplay.
'parasite' becomes 'flea' (I have seen 'Tiny parasite ' mean 'flea' so perhaps 'parasite' could also mean 'flea').
'back' says to take the final letters.
'to' means one lot of letters go next to another.
'sides' means to remove the middle letters (only the letters on the sides are used).
'change' with its centre taken out is 'ce'.
The last letter of 'ran' is 'n'.
'flea'+'n'+'ce'='FLEANCE'

'with' is the link.