Missionary’s motley crew, the size reduced (10)
I believe the answer is:
schweitzer
'missionary's' is the definition.
(philosopher and missionary Albert Schweitzer)
'motley crew the size reduced' is the wordplay.
'motley' indicates anagramming the letters (motley can mean varied or different).
'reduced' means to remove the last letter.
'crew'+'the'+'size'='crewthesize'
'crewthesize' with its last letter taken away is 'crewthesiz'.
'crewthesiz' anagrammed gives 'SCHWEITZER'.
(Other definitions for schweitzer that I've seen before include "Pursuing territorial or economic growth" , "See 19" , "theologian" , "Albert ---, Nobel peace laureate" , "Missionary" .)