One can bend vertically (4)
I believe the answer is:
unit
'one' is the definition.
('unit' can be a synonym of 'one')
'bend vertically' is the wordplay.
'bend' becomes 'u' (a U-bend is a kind of bend - pipe under sink).
'vertically' becomes 'nit' (I can't explain this - if you can you should give a lot more credence to this answer).
'u'+'nit'='UNIT'
'can' is the link.
Can you help me to learn more?
(Other definitions for unit that I've seen before include "Individual thing, single and complete" , "Separate part" , "Small military detachment" , "Standard for measuring" , "- - trust" .)