Thanks vgersh99
Here is the explanation

Code:

nawk -F"," 'FNR==NR{a["^" $1]=$2 OFS $3;next} {for (i in a) if ($2 ~ i) { print $1,$2,a[i];next}}' OFS="," file1 file2

Code:

FNR==NR

Checking for first file i.e file1

Code:

a["^" $1]=$2 OFS $3;next}

Creating array for first field of file1 with "^" appended to it
so array will be like
a[^cluster1]
a[^cluster2]
a[^cluster3]

Code:

next

the remaining code after next statement is not executed when awk is processing first file file1

Code:

{for (i in a) if ($2 ~ i) { print $1,$2,a[i];next}}

Code:

for (i in a)

looping through each element of array using awk special for loop

Code:

if ($2 ~ i)

checking for second field of file2 in each array index.
if found
print $1,$2,a[i];next

Code:

print $1,$2,a[i]

prints column 1 and 2 from file2 and array content

Code:

next

this is not needed.But doesnt hurt as well.

Code:

OFS=","

output field seperator is comma