Might of monarch supporting prisoner of war (5)
I believe the answer is:
power
'might' is the definition.
(being mighty)
'monarch supporting prisoner of war' is the wordplay.
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'prisoner of war' becomes 'POW' (synonyms).
'er' put after 'pow' is 'POWER'.
'of' is the link.
(Other definitions for power that I've seen before include "Control and influence exercised over others" , "Physical force, ability" , "Strength, force" , "Possession of controlling influence" , "Three to the third - is twenty-seven" .)