Pretend to be fit around fifty (5)
I believe the answer is:
bluff
'pretend' is the definition.
(thesaurus)
'fit around fifty' is the wordplay.
'fit' becomes 'buff' (**).
'around' means one lot of letters goes inside another.
'fifty' becomes 'l' ('L' can be a synonym of 'fifty').
'buff' enclosing 'l' is 'BLUFF'.
'to be' acts as a link.
(Other definitions for bluff that I've seen before include "Bank - subterfuge" , "Frank, direct; steep cliff" , "Deception - hearty" , "Act deceptively; steep cliff" , "Pretend - headland" .)