Entertainer's to demonstrate climbing large apparatus (8)
I believe the answer is:
showgirl
'entertainer's' is the definition.
(showgirl is a kind of entertainer)
'demonstrate climbing large apparatus' is the wordplay.
'demonstrate' becomes 'show' ('show' can be a synonym of 'demonstrate').
'climbing' is a reversal indicator (in a down clue, letters climb up).
'large' becomes 'l'.
'apparatus' becomes 'rig' (I've seen this before).
'l'+'rig'='lrig'
'lrig' written backwards gives 'girl'.
'show'+'girl'='SHOWGIRL'
'to' acts as a link.
(Another definition for showgirl that I've seen is " Entertainer".)