Observed man returning attacks (5)
I believe the answer is:
knees
'attacks' is the definition.
(to knee someone is to attack them with a knee)
'observed man returning' is the wordplay.
'observed' becomes 'seen' ('seen' can be a synonym of 'observed').
'man' becomes 'k' (abbreviation for king, a chess man).
'returning' is a reversal indicator.
'seen'+'k'='seenk'
'seenk' in reverse letter order is 'KNEES'.
(Other definitions for knees that I've seen before include "Lap" , "Legs' joints" , "Hinged joints" , "Hinge joints" , "Leg joints" .)