Use an array variable instead to store and execute your command - using a single string with literal double quotes won't be interpreted correctly:
# Store command tokens in an array.
# Note that each word or quoted phrase will form a separate array element.
cmd=( tshark -R "(ip.src == 10.129.5.192)" -r 12clients.pcap )
# Invoke command as the expanded array.
# Double-quoting it prevents shell expansions of the elements.
"${cmd[@]}"