How rook flapping by motorway gets a parasite (8)

'a parasite' is the definition.
(parasite that can live in a human's intestines)

'how rook flapping by motorway' is the wordplay.
'flapping' is an anagram indicator (flap can mean to move or fly around).
'by' says to put letters next to each other.
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'how'+'rook'='howrook'
'howrook' with letters rearranged gives 'hookwor'.
'hookwor'+'m'='HOOKWORM'

'gets' is the link.