Older relative with thing mostly for hard rock (7)
I believe the answer is:
granite
'hard rock' is the definition.
(I've seen this before)
'older relative with thing mostly' is the wordplay.
'older relative' becomes 'gran' (I've seen this before).
'with' means one lot of letters go next to another.
'thing' becomes 'it'.
'mostly' becomes 'e' (I can't justify this - if you can you should give a lot more credence to this answer).
'gran'+'it'+'e'='GRANITE'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for granite that I've seen before include "Hard stone from Tangier" , "Very hard type of rock" , "Showing great resolution" , "Tearing (anag.)" , "A very hard rock from Tangier" .)