Make cross after rotating length of ruler (6)
I believe the answer is:
regnal
'of ruler' is the definition.
(relating to a ruler or reign)
'make cross after rotating length' is the wordplay.
'make cross' becomes 'anger' (to anger someone is to make them cross).
'after rotating' says the letters should be written in reverse.
'length' becomes 'l' (abbreviation in geometry).
'anger' backwards is 'regna'.
'regna'+'l'='REGNAL'
(Other definitions for regnal that I've seen before include "about a rule" , "Relating to a monarch's reign" , "Angler (anag.); of a period in power" .)