The requirement to be less than knee deep (4)
I believe the answer is:
need
'the requirement' is the definition.
('need' can be a synonym of 'requirement')
'less than knee deep' is the wordplay.
'less' indicates a hidden word (the clue less some of the letters in it).
'NEED' is hidden within 'than knee deep'.
'to be' acts as a link.
(Other definitions for need that I've seen before include "Rely on" , "Cry out for - prerequisite" , "state of distress" , "What's lacking" , "state requiring relief" .)