Take the place of actor Christopher (around 51) (7)
I believe the answer is:
relieve
'take the place of' is the definition.
(I've seen this before)
'actor christopher around 51' is the wordplay.
'actor christopher' becomes 'reeve' (I've seen this in another clue).
'around' means one lot of letters goes inside another.
'51' becomes 'LI' (Roman numerals).
'reeve' enclosing 'li' is 'RELIEVE'.
(Other definitions for relieve that I've seen before include "Take over work from someone" , "Take over (from) - ease worries (of)" , "Set free" , "Ease pain or burden" , "cure" .)