Tin one penny? A bargain (4)
I believe the answer is:
snip
'a bargain' is the definition.
(slang term for a good deal)
'tin one penny?' is the wordplay.
'tin' becomes 'sn' (Sn is the chemical symbol for tin).
'one' becomes 'i' (Roman numeral**).
'penny?' becomes 'p'.
'sn'+'i'+'p'='SNIP'
(Other definitions for snip that I've seen before include "Bargain (coll)" , "Trim, clip" , "Cut - prune" , "bargain buy" , "Cut quickly with a scissors" .)