Question

Can a clerk call an extraordinary meeting, standing order say a chairman can or two councillors etc

Answer 1

No.  Its Chairman or (if he/she fails to do so) 2 Councillors.

Comments

A chair may do so of their own volition, or if 2 cllrs submit a written request there may be an extraordinary meeting.
It gets a bit more granular after that.
If, on receipt of 2x written requests a chair fails to convene a meeting within 7 days, 4 cllrs may compile, sign and issue their own agenda.
The difficulty with that is - convene only means name the date, so a chair ‘could’ name a date weeks hence so long as they do so within 7 days of receipt of the x2 written requests.
The regs are a bit of a nonsense in that regard.