Sits astride skylight casing over wobbly ladders (9)
I believe the answer is:
straddles
'sits astride' is the definition.
(as in straddling a horse)
'skylight casing over wobbly ladders' is the wordplay.
'casing' says to hollow out the word (remove centre letters) (the outside letters are the case of the word).
'over' says to put letters next to each other.
'wobbly' is an anagram indicator (I've seen this in other clues).
'skylight' with its centre taken out is 'st'.
'ladders' is an anagram of 'raddles'.
'st'+'raddles'='STRADDLES'
(Other definitions for straddles that I've seen before include "Stands across" , "Sits astride" .)