Agree it’s a reason for not getting a mongrel (6)
I believe the answer is:
concur
'agree' is the definition.
(I know that agree can be written as concur)
'not getting a mongrel' is the wordplay.
'not' becomes 'con' (I've seen this in other clues).
'getting' says to put letters next to each other.
'a mongrel' becomes 'cur' ('cur' can be a synonym of 'mongrel').
'con'+'cur'='CONCUR'
'it's a reason for' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for concur that I've seen before include "share the same opinion" , "'Agree, coincide (6)'" , "Be in accord" , "Express agreement" , "run together" .)