Prince with severe expression covering front of curious old coin (4-5)
I believe the answer is:
half-crown
'old coin' is the definition.
(historical type of coin)
'prince with severe expression covering front of curious' is the wordplay.
'prince' becomes 'hal' (I've seen this before).
'with' means one lot of letters go next to another.
'severe expression' becomes 'frown' (severe facial expression).
'covering' means one lot of letters goes inside another.
'front of' indicates taking the first letters.
The first letter of 'curious' is 'c'.
'frown' going around 'c' is 'fcrown'.
'hal'+'fcrown'='HALF-CROWN'
(Other definitions for half-crown that I've seen before include "Former British coin" , "Old British coin" , "Former silver coin" .)