Crossing centre, I sit on rocks (12)
I believe the answer is:
intersection
'crossing' is the definition.
(synonyms)
'centre i sit on rocks' is the wordplay.
'rocks' indicates an anagram.
'centre'+'i'+'sit'+'on'='centreisiton'
'centreisiton' anagrammed gives 'INTERSECTION'.
(Other definitions for intersection that I've seen before include "Motorway junction" , "Crossing point" , "Where two lines cross" , "Crossing - junction" .)