Tom Tiddler? (7)
I believe the answer is:
catfish
'tom tiddler?' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'tom tiddler?' is the wordplay.
'tom' becomes 'cat' (a tom is a male cat).
'tiddler?' becomes 'fish' (I can't justify this - if you can you should believe this answer much more).
'cat'+'fish'='CATFISH'
Can you help me to learn more?
(Other definitions for catfish that I've seen before include "Bewhiskered swimmer?" , "Swimmer with whiskery barbels" , "His fact (anag)" , "Freshwater type" , "Aquatic creature with barbels" .)