Sent elastic strap to go round our uniform pants (9)
I believe the answer is:
rapturous
'sent' is the definition.
(thesaurus)
'elastic strap to go round our uniform pants' is the wordplay.
'elastic' is an anagram indicator (the anagram stretches or twists into a new form).
'to go round' means one lot of letters goes inside another.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'pants' indicates an anagram (UK informal term for 'nonsense' or 'rubbish').
'strap' anagrammed gives 'rapts'.
'our'+'u'='ouru'
'ouru' is an anagram of 'urou'.
'rapts' placed around 'urou' is 'RAPTUROUS'.
(Other definitions for rapturous that I've seen before include "Blissful" , "Wildly" , "Feeling great pleasure" , "over the moon" , "Ecstatic" .)