Honesty of Rob overwhelmed by compassion (7)
I believe the answer is:
probity
'honesty' is the definition.
(probity is a kind of honesty)
'rob overwhelmed by compassion' is the wordplay.
'overwhelmed by' is an insertion indicator (I've seen this in other clues).
'compassion' becomes 'pity' (pity is a kind of compassion).
'rob' inserted within 'pity' is 'PROBITY'.
'of' acts as a link.
(Other definitions for probity that I've seen before include "uprightness" , "integrity's demanded" , "Honesty, decency" , "Integrity" .)