Bell, British soldier: well out of it (3)
I believe the answer is:
tom
'bell' is the definition.
(I've seen this before)
'british soldier well out of it' is the wordplay.
'british soldier' becomes 'tommy' (I've seen this before).
'well' becomes 'my' (both are expressions of surprise).
'out of it' indicates named letters should be taken away.
'tommy' with 'my' removed is 'TOM'.
(Other definitions for tom that I've seen before include "Edgar's Poor disguise (Lear)" , "Male feline" , "Hoyden" , "Waits, perhaps" , "He is associated with Dick and Harry" .)