Determined to add tees correctly (4,3)
I believe the answer is:
dead set
'determined to' is the definition.
(synonyms)
'add tees correctly' is the wordplay.
'correctly' is an anagram indicator.
'add'+'tees'='addtees'
'addtees' is an anagram of 'DEAD-SET'.
(Other definitions for dead set that I've seen before include "Absolutely determined, bent on" , "Resolute -- sedated (anag)" , "A determined attack - from extinct group?" , "A determined attack, sounds moribund" , "A determined attack - from moribund group?" .)