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 can't justify this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'a form of' indicates anagramming the letters (another form of the same letters).
'grid' with letters rearranged gives 'rigd'.
'i' going 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" .)