Reason guitar clamps fit centrally (5)
I believe the answer is:
basis
'reason' is the definition.
(I've seen this before)
'guitar clamps fit centrally' is the wordplay.
'guitar clamps' becomes 'bas' (I can't explain this - if you can you should believe this answer much more).
'fit' becomes 'commission' (commissioning is a kind of fitting).
'centrally' indicates the central letters.
The central letters of 'commission' are 'is'.
'bas'+'is'='BASIS'
Can you help me to learn more?
(Other definitions for basis that I've seen before include "Principle on which a theory is founded" , "grounds" , "Foundation or support, or a main principle" , "Underlying support for a theory" , "Ground, foundation" .)