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 odd-numbered positions).
'in' indicates putting letters inside.
'cunning plan' becomes 'ruse' (I've seen this before).
The alternate letters of 'excels' are 'ecl'.
'ecl' going 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" .)