Goodness! Monsieur has raised flag! (5)
I believe the answer is:
merit
'goodness' is the definition.
(I've seen this before)
'monsieur has raised flag' is the wordplay.
'monsieur' becomes 'm'.
'has' means one lot of letters go next to another.
'raised' shows that the letters should be reversed in order.
'flag' becomes 'tire' (I've seen this before).
'tire' back-to-front is 'erit'.
'm'+'erit'='MERIT'
(Other definitions for merit that I've seen before include "The M in OM" , "Value" , "Earn" , "Just deserts" , "Goodness" .)