Crucial factor in last row (6,4)
I believe the answer is:
bottom line
'crucial factor' is the definition.
'bottom line' can be an answer for 'factor' (I have seen 'Fundamental factor ' mean 'bottom line' so perhaps 'factor' could also mean 'bottom line'). I am unsure of the 'crucial' bit.
'last row' is the wordplay.
'last' becomes 'bottom' (I've seen this before**).
'row' becomes 'line' (I've seen this before).
'bottom'+'line'='BOTTOM LINE'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for bottom line that I've seen before include "deciding factor" , "location of this answer" , "Fundamental factor" , "The basic reality of a situation" , "Profit or loss" .)