Compassionate? Lucifer, surprisingly, after mass! (8)
I believe the answer is:
merciful
'compassionate?' is the definition.
(synonyms)
'lucifer surprisingly after mass' is the wordplay.
'surprisingly' indicates an anagram.
'after' means one lot of letters go next to another.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'lucifer' anagrammed gives 'erciful'.
'erciful' after 'm' is 'MERCIFUL'.
(Other definitions for merciful that I've seen before include "Kind" , "Clement - forbearing" , "Humane" , "Showing pity or leniency" , "Compassionate, forgiving" .)