Through which a moth ingests a bit of grub (5)
I believe the answer is:
mouth
'ingests a bit of grub' is the definition.
I can't judge whether this definition defines the answer.
'through which a moth' is the wordplay.
'through' is an insertion indicator.
'which' becomes 'u' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'u' going within 'moth' is 'MOUTH'.
Can you help me to learn more?
(Other definitions for mouth that I've seen before include "River part - conversation opener?" , "Gob" , "Oral cavity" , "Where a river enters the sea" , "trap" .)