Manager left in regret before start of relegation (5)
I believe the answer is:
ruler
'manager' is the definition.
(thesaurus)
'left in regret before start of relegation' is the wordplay.
'left' becomes 'l' (common abbreviation**).
'in' indicates putting letters inside.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'before' says to put letters next to each other.
'start of' indicates taking the first letters.
The first letter of 'relegation' is 'r'.
'l' put into 'rue' is 'rule'.
'rule'+'r'='RULER'
(Other definitions for ruler that I've seen before include "Measuring device" , "Governor or drawing stick" , "' - - of the Queen's Navee!'" , "Graduated straight-edge" , "Drawing stick or monarch" .)