Invest £1000 in a promising merchantman (6)
I believe the answer is:
argosy
'invest ps1000' is the definition.
I can't judge whether this defines the answer.
'in a promising merchantman' is the wordplay.
'in' indicates putting letters inside.
'promising' becomes 'rosy' ('rosy' can be a synonym of 'promising').
'merchantman' becomes 'g' (I can't explain this - if you can you should give a lot more credence to this answer).
'a'+'rosy'='arosy'
'arosy' going around 'g' is 'ARGOSY'.
Can you help me to learn more?
(Other definitions for argosy that I've seen before include "Ancient merchant ship" , "A great fleet as Spain once had" , "One or more large merchant ships" , "Large merchant ship (poetic)" , "merchant fleet" .)