Second-rate relationship is over very quickly (5)
I believe the answer is:
blink
'over very quickly' is the definition.
I can't tell whether this defines the answer.
'second-rate relationship' is the wordplay.
'second' becomes 'b' (B can mean second eg B-side).
'rate relationship' becomes 'link' (I can't explain this - if you can you should give a lot more credence to this answer).
'b'+'link'='BLINK'
'is' is the link.
Can you help me to learn more?
(Other definitions for blink that I've seen before include "Briefly shut the eyes" , "Rapid eye movement" , "Brief moment (of hesitation?)" , "viewing interruption" , "Nictate" .)