Pair helping one see incomplete requirement (4)
I believe the answer is:
spec
'requirement' is the definition.
(I've seen this before)
'pair helping one see incomplete' is the wordplay.
'pair helping one see' becomes 'specs' (pair of spectacles).
'incomplete' means to remove the last letter.
'specs' with its last letter taken off is 'SPEC'.
(Other definitions for spec that I've seen before include "Instructions" , "on which chance is taken" , "Minor description of detail" , "On - means on the chance of achieving something" , "On - means without a plan but with hope of success" .)