Crustacean thus invading one school of whales (6)
I believe the answer is:
isopod
'crustacean' is the definition.
(isopod is a kind of crustacean)
'thus invading one school of whales' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'invading' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'school of whales' becomes 'pod' (I've seen this before).
'i'+'pod'='ipod'
'so' inserted within 'ipod' is 'ISOPOD'.
(Other definitions for isopod that I've seen before include "This crusty creature" , "Woodlouse, say" , "Crustacean with seven pairs of legs for crawling" , "Crustacean such as a woodlouse" , "Slater, for instance" .)