Made fast run during low-energy day (6)
I believe the answer is:
moored
'made fast' is the definition.
(mooring is a kind of making fast)
'run during low-energy day' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'during' indicates putting letters inside.
'low' becomes 'moo' (both can mean to make a cow noise).
'energy' becomes 'e' (physics symbol as in E=mc2).
'day' becomes 'd' (abbreviation for day).
'moo'+'e'='mooe'
'r' put inside 'mooe' is 'moore'.
'moore'+'d'='MOORED'
(Other definitions for moored that I've seen before include "Fastened with a line or anchored" , "Of boat, secured by rope to shore" , "Secured by rope or cable" , "Made fast" , "Parked (of a boat)" .)