Something to fill gaps when mammal and insect meet (7)
I believe the answer is:
sealant
'something to fill gaps' is the definition.
I can't tell whether this defines the answer.
'mammal and insect meet' is the wordplay.
'mammal' becomes 'seal' (type of mammal).
'and' says to put letters next to each other.
'insect meet' becomes 'ant' (I can't justify this - if you can you should give a lot more credence to this answer).
'seal'+'ant'='SEALANT'
'when' acts as a link.
Can you help me to learn more?
(Other definitions for sealant that I've seen before include "closer?" , "Material to prevent leakage" , "Bond" , "Substance for closing something tightly" , "Material stopping leaks" .)