Tommy’s weapon close to mysterious figure (4)
I believe the answer is:
sten
'tommy's weapon' is the definition.
(I've seen this in another clue)
'close to mysterious figure' is the wordplay.
'close to' indicates one should take the final letters (the close/ending of).
'figure' becomes 'ten' (ten is a number or figure).
The last letter of 'mysterious' is 's'.
's'+'ten'='STEN'
(Other definitions for sten that I've seen before include "Deadly weapon" , "name of gun" , "One may be trained" , "dart of pain to Rob Roy, say" , "Light sub-machine (gun)" .)