Value in know-how, or theory (5)
I believe the answer is:
worth
'value' is the definition.
(I've seen this before)
'in know-how or theory' is the wordplay.
'in' indicates the answer is hidden within the clue.
'WORTH' can be found hidden inside 'know how or theory'.
(Other definitions for worth that I've seen before include "Throw (anag.)" , "Merit - value" , "Value (of something)" .)