Undeviating line in a form of grid (5)
I believe the answer is:
rigid
'undeviating' is the definition.
(thesaurus)
'line in a form of grid' is the wordplay.
'line' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'in' is an insertion indicator.
'a form of' indicates anagramming the letters (the letters need to be in another 'form').
'grid' is an anagram of 'rigd'.
'i' put within 'rigd' is 'RIGID'.
Can you help me to learn more?
(Other definitions for rigid that I've seen before include "strict" , "Uncompromising" , "Stiff, unyielding" , "Incapable of or resistant to bending" , "completely" .)