In there, perhaps, one offers a choice (6)
I believe the answer is:
either
'a choice' is the definition.
(I have seen 'Choice of two' mean 'either' so perhaps 'choice' could also mean 'either')
'in there perhaps one' is the wordplay.
'in' indicates putting letters inside.
'perhaps' indicates anagramming the letters.
'one' becomes 'i' (Roman numeral).
'there' with letters rearranged gives 'ether'.
'ether' enclosing 'i' is 'EITHER'.
'offers' is the link.
(Other definitions for either that I've seen before include "offered a choice" , "could be one of two" , "Choice of two" , "Word indicating the following alternatives" , "Each of two; moreover" .)