That is restricting grandparent - it's senseless (5)
I believe the answer is:
inane
'senseless' is the definition.
(I've seen this before)
'that is restricting grandparent' is the wordplay.
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'restricting' indicates putting letters inside.
'grandparent' becomes 'nan' (I can't justify this - if you can you should give a lot more credence to this answer).
'ie' going around 'nan' is 'INANE'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for inane that I've seen before include "Dull-witted" , "Fatuous, mindless" , "Foolish, fatuous" , "unimaginative" , "Thick" .)