Smell bottom of toddler — it's alarming! (4)
I believe the answer is:
reek
'smell' is the definition.
(reek is a kind of smell)
'bottom of toddler it's alarming' is the wordplay.
'bottom of' indicates one should take the final letters.
'it's alarming' becomes 'eek' (I am not sure about this - if you are sure you should believe this answer much more).
The last letter of 'toddler' is 'r'.
'r'+'eek'='REEK'
Can you help me to learn more?
(Other definitions for reek that I've seen before include "Fume" , "Smell like 12 Across" , "Smell of smoke in Kerry?" , "terrible smell" , "Smell, like a Kerry mountain?" .)