Assess the depth of (5)
I believe the answer is:
plumb
'assess the depth of' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for plumb that I've seen before include "Lead weight; exactly" , "True" , "Reach or experience, say the depths" , "Weight to show vertical direction" , "Check depth of water below" .)