Fundamental part of fabric is absent, on reflection (5)
I believe the answer is:
basic
'fundamental' is the definition.
('basic' can be similar in meaning to 'fundamental')
'part of fabric is absent on reflection' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'on reflection' says the letters should be written in reverse.
'cisab' is hidden within 'fabric is absent'.
'cisab' backwards is 'BASIC'.
(Other definitions for basic that I've seen before include "Introductory" , "Spartan" , "Without frills" , "Forming a foundation" , "Popular programming language" .)