Oddly neat ninja becomes grandparent (5)
I believe the answer is:
nanna
'grandparent' is the definition.
(I know that nanna is a type of grandparent)
'oddly neat ninja' is the wordplay.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
'neat'+'ninja'='neatninja'
The alternating letters of 'neatninja' are 'NANNA'.
'becomes' is the link.
(Other definitions for nanna that I've seen before include "Grandma" , "Grandmother (colloquial)" , "Senior woman" , "old Indian currency" , "Mother's mother?" .)