Second stone collected by relative cracks it open (7)
I believe the answer is:
instant
'second stone' is the definition.
'instant' can be an answer for 'second' (I've seen this before). I'm unsure of the 'stone' bit.
'collected by relative cracks it open' is the wordplay.
'collected by' indicates putting letters inside.
'relative' becomes 'nan' (I have seen 'Elderly relative' mean 'nan' so perhaps 'relative' could also mean 'nan').
'cracks' means one lot of letters goes inside another (some letters create an opening in the word).
'open' becomes 'st' (I am not sure about this - if you are sure you should believe this answer much more).
'nan' going within 'it' is 'inant'.
'inant' placed around 'st' is 'INSTANT'.
(Other definitions for instant that I've seen before include "Without delay - short time" , "Jiffy" , "pressing" , "Split second" , "Short moment" .)