Minute yellow rodent returned — it’s in many walls (6)
I believe the answer is:
mortar
'it's in many walls' is the definition.
I can't judge whether this defines the answer.
'minute yellow rodent returned' is the wordplay.
'minute' becomes 'm' (abbreviation).
'yellow' becomes 'or' (heraldic term).
'rodent' becomes 'rat' (rat is a kind of rodent).
'returned' shows that the letters should be reversed in order.
'rat' in reverse letter order is 'tar'.
'm'+'or'+'tar'='MORTAR'
Can you help me to learn more?
(Other definitions for mortar that I've seen before include "Bond for bricks; weapon" , "Small cannon or building material" , "Brick bonding mixture" , "Weapon - bonding material" , "Used in building a house - or to destroy one" .)