He infiltrated the mob, metamorphosing into a monster (8)
I believe the answer is:
behemoth
'a monster' is the definition.
(I know that monster can be written as behemoth)
'he infiltrated the mob metamorphosing' is the wordplay.
I cannot quite see how this works, but
'he' could be 'em' and 'em' is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
This explanation may well be incorrect...
'into' acts as a link.
Can you help me to learn more?
(Other definitions for behemoth that I've seen before include "Giant" , "Anything abnormally large and powerful" , "Monstrous beast, or thing" , "Big corporation" , "Abnormally large creature" .)