Rarely has one topic that one can crack (9)
I believe the answer is:
pistachio
'that one can crack' is the definition.
(a pistachio nut can be cracked)
'rarely has one topic' is the wordplay.
'rarely' indicates anagramming the letters (rare can mean unusual or strange).
'one' becomes 'I' (Roman numeral).
'has'+'i'+'topic'='hasitopic'
'hasitopic' with letters rearranged gives 'PISTACHIO'.
(Other definitions for pistachio that I've seen before include "Nut - it is a chop (anag)" , "light green" , "nut-bearing tree" , "Edible pale green seed" , "Nut tree" .)