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' means one lot of letters goes inside another (some letters get engaged in others).
'immediately' becomes 'stat' (short for Latin statim).
'cic' enclosing '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" .)