Look into mantis, for one, devouring partner's head (7)
I believe the answer is:
inspect
'look into' is the definition.
(I've seen this before)
'mantis for one devouring partner's head' is the wordplay.
'mantis for one' becomes 'insect' (mantis is a kind of insect).
'devouring' is an insertion indicator.
'partner's head' becomes 'p' (I am not sure about this - if you are sure you should believe this answer much more).
'insect' enclosing 'p' is 'INSPECT'.
Can you help me to learn more?
(Other definitions for inspect that I've seen before include "test" , "Ecamine closely" , "Go over" , "Study" , "Scan" .)