Lively trip -- side let loose (8)
I believe the answer is:
spirited
'lively' is the definition.
('spirited' can be similar in meaning to 'lively')
'trip side let loose' is the wordplay.
'let loose' indicates anagramming the letters.
'trip'+'side'='tripside'
'tripside' is an anagram of 'SPIRITED'.
(Other definitions for spirited that I've seen before include "Riptides of lively kind" , "Secretly transported" , "Lively and brave" , "Vigorous" , "Mettlesome" .)