Significant runs in good collection of openers (3,4)
I believe the answer is:
key ring
'collection of openers' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'significant runs in good' is the wordplay.
'significant' becomes 'key' (I've seen this in another clue).
'runs' becomes 'r' (cricket abbreviation).
'good' becomes 'g' (abbreviation).
'key'+'r'+'in'+'g'='KEY-RING'
Can you help me to learn more?
(Other definitions for key ring that I've seen before include "Gaoler's holder" , "Grey ink (anag)" , "place for lockers" , "a lot may hang on it" , "What may carry a fob" .)