Harshness when monarch engaged in compassion (8)
I believe the answer is:
asperity
'harshness' is the definition.
(thesaurus)
'when monarch engaged in compassion' is the wordplay.
'when' becomes 'as'.
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'engaged in' means one lot of letters goes inside another.
'compassion' becomes 'pity' (pity is a kind of compassion).
'er' going within 'pity' is 'perity'.
'as'+'perity'='ASPERITY'
(Other definitions for asperity that I've seen before include "Harshness or severity of manner" , "Roughness, bitterness" , "Harshness, bitter coldness" , "Irritability" , "Roughness of temper" .)