Make changes to stop allegation restricting old scientist (15)
I believe the answer is:
palaeontologist
'scientist' is the definition.
(palaeontologist is a kind of scientist)
'make changes to stop allegation restricting old' is the wordplay.
'make changes to' indicates an anagram.
'restricting' indicates putting letters inside.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'stop'+'allegation'='stopallegation'
'stopallegation' anagrammed gives 'palaeontolgist'.
'palaeontolgist' placed around 'o' is 'PALAEONTOLOGIST'.
(Another definition for palaeontologist that I've seen is " one found in Natural History Museum?".)