To be in charge will make a fellow grow old (6)
I believe the answer is:
manage
'to be in charge' is the definition.
(I've seen this in another clue)
'a fellow grow old' is the wordplay.
'a fellow' becomes 'man' (fellow can mean a man).
'grow old' becomes 'age' (synonyms).
'man'+'age'='MANAGE'
'will make' is the link.
(Other definitions for manage that I've seen before include "Run, oversee" , "Control; cope" , "Handle, cope with" , "Run, be in charge of" , "Be in charge of something" .)