xyz()
{
x=$1*2
echo x
}

abc,2,something
def,3,something1

abc,4,somthing
def,6,something1

cat input.csv|awk -F, -v v="'"`xyz "$2""'" 'BEGIN {FS=","; OFS=","} {$2=v1; print $0}'

cat input.csv|awk -F, -v v="'"`xyz "14""'" 'BEGIN {FS=","; OFS=","} {$2=v1; print $0}'

cat input.csv|awk -F, -v v="'"`xyz "$2""'" 'BEGIN {FS=","; OFS=","} {$2=v1; print $0}'
                         ^ Here        ^ Here                            ^ Here

awk -F, -v v="'$(xyz "$2")'" 'BEGIN {FS=","; OFS=","} {$2=v; print $0}' input.csv

awk -F, 'BEGIN { FS = ","; OFS = "," }
         function xyz(a) { return a * 2 }
         { $2 = xyz($2); print $0 }'
    input.csv