Get back ruler overthrown by power grab (6)
I believe the answer is:
recoup
'get back' is the definition.
(as in recouping funds)
'ruler overthrown by power grab' is the wordplay.
'ruler' becomes 'ER' (abbreviation for Elizabeth Regina).
'overthrown' is a reversal indicator.
'by' says to put letters next to each other.
'power grab' becomes 'coup' (I've seen this in another clue).
'er' back-to-front is 're'.
're'+'coup'='RECOUP'
(Other definitions for recoup that I've seen before include "Get back again, say losses (6)" , "Get back, say expenses" , "Recover, losses perhaps" , "Recover - retrieve" , "Get back one's losses" .)