Sent English Commander-in-Chief to engage immediately (8)
I believe the answer is:
ecstatic
'sent' is the definition.
(thesaurus)
'english commander-in-chief to engage immediately' is the wordplay.
'english' becomes 'E' (abbreviation).
'commander-in-chief' becomes 'CIC' (abbreviation).
'to engage' is an insertion indicator (some letters get engaged in others).
'immediately' becomes 'stat' (short for Latin statim).
'cic' placed around 'stat' is 'cstatic'.
'e'+'cstatic'='ECSTATIC'
(Other definitions for ecstatic that I've seen before include "In religious fervour" , "Emotionally exalted" , "Feeling great rapture or delight" , "Cites act about being thrilled to bits" , "On cloud nine" .)