Unusually powerful King restrained on losing son (6)
I believe the answer is:
oberon
'unusually powerful king' is the definition.
(fairy king in A Midsummer Night's Dream)
'restrained on losing son' is the wordplay.
'restrained' becomes 'sober' (I've seen this before).
'losing' is a deletion indicator.
'son' becomes 's' (genealogical abbreviation for son).
'sober'+'on'='soberon'
'soberon' with 's' removed is 'OBERON'.
(Other definitions for oberon that I've seen before include "King of the fairies (AMND)" , "Shakespearean role" , "MND fairy king" , "Shakespeare's fairy king" , "Moon" .)