Kind of dome on the allotment? (5)
I believe the answer is:
onion
'kind of dome' is the definition.
(I've seen this in another clue)
'on the allotment?' is the wordplay.
'the allotment?' becomes 'ion' (I can't explain this - if you can you should believe this answer much more).
'on'+'ion'='ONION'
Can you help me to learn more?
(Other definitions for onion that I've seen before include "Shape of Russian dome" , "reason for tears" , "Hot dog topping" , "Head (slang); vegetable" , "Strong tasting vegetable" .)