Tend or train mouse, say (6)
I believe the answer is:
rodent
'mouse say' is the definition.
(I know that mouse is a type of rodent)
'tend or train' is the wordplay.
'train' indicates an anagram (I've seen 'trained' mean this (to train can mean to grow something into a shape)).
'tend'+'or'='tendor'
'tendor' anagrammed gives 'RODENT'.
(Other definitions for rodent that I've seen before include "Toothy mammal" , "Animal such as a mouse" , "E.g. hamster" , "Mouse, squirrel or rat" , "Nibbler" .)