Fixed line in a grid, perhaps (5)
I believe the answer is:
rigid
'fixed' is the definition.
(I know that fixed can be written as rigid)
'line in a grid perhaps' 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' is an insertion indicator.
'perhaps' indicates anagramming the letters.
'grid' is an anagram of 'rigd'.
'i' placed within 'rigd' is 'RIGID'.
Can you help me to learn more?
(Other definitions for rigid that I've seen before include "strict" , "Uncompromising" , "stern" , "Frozen" , "completely" .)