Space to drop anchor mounted under barge (5,4)
I believe the answer is:
elbow room
'space' is the definition.
(elbow room can mean space to move)
'drop anchor mounted under barge' is the wordplay.
'drop anchor' becomes 'moor' (I've seen this in another clue).
'mounted' is a reversal indicator.
'under' says to put letters next to each other.
'barge' becomes 'elbow' (elbow can mean to barge or nudge out the way).
'moor' back-to-front is 'room'.
'room' after 'elbow' is 'ELBOW-ROOM'.
'to' is the link.
(Other definitions for elbow room that I've seen before include "find adequate space to work in" , "Space to move" , "play" , "Space for manoeuvre" , "Freedom to move" .)