Laid-back cardinal holding large guitar (7)
I believe the answer is:
relaxed
'laid-back' is the definition.
(similar in meaning)
'cardinal holding large guitar' is the wordplay.
'cardinal' becomes 'red' (cardinal is a shade of red).
'holding' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'guitar' becomes 'axe' (informal term for a guitar).
'l'+'axe'='laxe'
'red' placed around 'laxe' is 'RELAXED'.
(Other definitions for relaxed that I've seen before include "Rested" , "Chilled; made less strict" , "not uptight" , "Having put one's feet up" , "composed" .)