In Boston, pop old penny into it (4)
I believe the answer is:
soda
'in boston pop' is the definition.
'soda' can be an answer for 'pop' (fizzy pop). I'm unsure of the 'in boston' bit.
'old penny into it' is the wordplay.
'old' becomes 'o' (abbreviation).
'penny' becomes 'd' (abbreviation e.g. Lsd - pounds, shillings, pence).
'into' is an insertion indicator.
'it' becomes 'sa' (old-fashioned abbreviation for sex appeal).
'o'+'d'='od'
'od' put inside 'sa' is 'SODA'.
Can you help me to learn more?
(Other definitions for soda that I've seen before include "Splash served with some spirits" , "Alkaline substance used in cleaning, drinks etc" , "Soft drink" , "Whisky mixer" , "pressurised liquid" .)