Shareholder wearing undergarment as an alternative (8)
I believe the answer is:
investor
'shareholder' is the definition.
(I've seen this before)
'wearing undergarment as an alternative' is the wordplay.
'wearing' becomes 'in' (you are 'in' the clothes you're wearing).
'undergarment' becomes 'vest' (vest is a kind of undergarment).
'as an alternative' becomes 'or' ('or' introduces an alternative).
'in'+'vest'+'or'='INVESTOR'
(Other definitions for investor that I've seen before include "Eg, buyer of stocks" , "market trader?" , "city speculator, perhaps" , "potential moneymaker" , "One putting money into shares, etc" .)