Dog eating two thousand caterpillars? Only one! (7)
I believe the answer is:
cutworm
'caterpillars? only one' is the definition.
I can't tell whether this defines the answer.
'dog eating two thousand' is the wordplay.
'dog' becomes 'cur' (cur is a kind of dog).
'eating' indicates putting letters inside.
'thousand' becomes 'm' (synonyms).
'cur' enclosing 'two' is 'cutwor'.
'cutwor'+'m'='CUTWORM'
Can you help me to learn more?
(Other definitions for cutworm that I've seen before include "pernicious type" , "Form of insect" , "Caterpillar" .)