Regret carrying large measuring stick (4)
I believe the answer is:
rule
'measuring stick' is the definition.
(rule is a kind of measuring stick)
'regret carrying large' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'carrying' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'rue' going around 'l' is 'RULE'.
(Other definitions for rule that I've seen before include "Exercise authority over" , "Govern with straight stick?" , "Standard" , "Regulation, control" , "Prescribed guide for conduct" .)