One lacks a full compliment of members (7)
I believe the answer is:
amputee
'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'lacks a full compliment of members' is the wordplay.
I cannot really see how this works, but
'a' could be 'amp' (A is the symbol for the amp or ampere) and 'amp' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for amputee that I've seen before include "One who has limb surgically removed" , "Person with limb(s) removed" , "Possible Paralympian" , "Person who has had limb removed surgically" , "One with missing limb" .)