Convenient and extremely hardy on the outside (5)
I believe the answer is:
handy
'convenient' is the definition.
(similar in meaning)
'and extremely hardy on the outside' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'on the outside' indicates putting letters inside.
'hardy' with its middle removed is 'hy'.
'and' inserted into 'hy' is 'HANDY'.
(Other definitions for handy that I've seen before include "Accessible" , "Near; useful" , "Useful - competent" , "Convenient; skilful" , "Useful and convenient" .)