Bring to bear on a former royal titular head (5)
I believe the answer is:
exert
'bring to bear' is the definition.
(I've seen this before)
'former royal titular head' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'royal' becomes 'ER' (abbreviation for Elizabeth Regina).
'head' says to take the initial letters.
The initial letter of 'titular' is 't'.
'ex'+'er'+'t'='EXERT'
'on a' is the link.
(Other definitions for exert that I've seen before include "Bring (force) to bear" , "Exercise, wield" , "Work very hard" , "Bring quality or influence into play" , "Bring into play, wield" .)