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 explain 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 (the letters need to be in another 'form').
'grid' with letters rearranged gives 'rigd'.
'i' going inside '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" .)