It's incomprehensible to Tommy! (3)
I believe the answer is:
rot
'tommy' is the definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?
'it's incomprehensible to' is the wordplay.
'it' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'incomprehensible' indicates an anagram (an incomprehensible jumbling of the letters).
'to' with letters rearranged gives 'ot'.
'r'+'ot'='ROT'
Can you help me to learn more?
(Other definitions for rot that I've seen before include "Paradise" , "Gradual decline" , "Decay in grotesque way" , "Deterioration" , "Decay or disintegrate" .)