Loner excels oddly in cunning plan (7)
I believe the answer is:
recluse
'loner' is the definition.
(I know that recluse is a type of loner)
'excels oddly in cunning plan' is the wordplay.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
'in' is an insertion indicator.
'cunning plan' becomes 'ruse' (I've seen this before).
The alternating letters of 'excels' are 'ecl'.
'ecl' put into 'ruse' is 'RECLUSE'.
(Other definitions for recluse that I've seen before include "Solitary person who avoids others" , "Person who lives alone and avoids others" , "Miss Havisham" , "One who lives alone avoiding people" , "Person living in solitude" .)