Crossword Genius

Royal portrait for a penny, perhaps (5)

Ross

I believe the answer is:

stamp

I'm a little stuck... Click here to teach me more about this clue!

'royal portrait' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

'a penny perhaps' is the wordplay.
I cannot really understand how this works, but
'a' could be 'amp' (A is the symbol for the amp or ampere) and 'amp' is located in the answer.
The remaining letters 'st' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

'for' is the link.

Can you help me to learn more?

(Other definitions for stamp that I've seen before include "Use a die; character" , "What to put on 7 before putting in 15" , "cast?" , "Seal" , "making an impression?" .)

I've seen this clue in the Evening Standard.
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