Tie down unruly set in the cooler (6)
I believe the answer is:
fasten
'tie down' is the definition.
(thesaurus)
'unruly set in the cooler' is the wordplay.
'unruly' is an anagram indicator.
'in' indicates putting letters inside.
'the cooler' becomes 'fan' (something which cools).
'set' with letters rearranged gives 'ste'.
'ste' going inside 'fan' is 'FASTEN'.
(Other definitions for fasten that I've seen before include "Fix -- make tight" , "Tie, secure" , "Shut" , "Fix to, secure" , "Bolt" .)