Create delta column - pkt timestamp & FIX SendingTime

Could someone let me know if it's possible to create a column that displays the delta between a packet's arrival time and the FIX SendingTime in the payload?

Unfortunately that's not possible without (a rather big) change of the Wireshark code.

I have the two timestamps in adjacent columns - just not sure how to add a delta column.

You could 'print' the columns you need with tshark and then use a script (perl, python, etc.) to calculate the delta time.

tshark -nr FIX.cap -T fields -e frame.number -e frame.time -e fix.SendingTime -e ip.src -e ip.dst -E separator=;

or

tshark -nr FIX.cap -T fields -e frame.number -e frame.time_epoch -e fix.SendingTime -e ip.src -e ip.dst -E separator=;

Regards
Kurt