Toy dog has to clean up (3)
I believe the answer is:
pom
'toy dog' is the definition.
'pom' can be an answer for 'dog' (short for Pomeranian). I am not sure about the 'toy' bit.
'to clean up' is the wordplay.
'to clean' becomes 'mop' (mopping is a kind of cleaning).
'up' is a reversal indicator (in down clue: letters go upwards).
'mop' in reverse letter order is 'POM'.
'has' is the link.
Can you help me to learn more?
(Other definitions for pom that I've seen before include "Little dog" , "Type of dog" , "Englishman in Australia" , "English person" , "Australian term for Englishman" .)