Try swallowing the whole onion! (7)
I believe the answer is:
shallot
'onion' is the definition.
(I know that shallot is a type of onion)
'try swallowing the whole' is the wordplay.
'try' becomes 'shot' (both can mean an attempt).
'swallowing' is an insertion indicator.
'the whole' becomes 'all' (all of something).
'shot' going around 'all' is 'SHALLOT'.
(Other definitions for shallot that I've seen before include "Edible onion-like bulb" , "Small onion" , "Scallion" , "found in the allotment" , "Small onion-like bulb used for seasoning" .)