Before close of play, big match is liable to boil over (5)
I believe the answer is:
testy
'boil over' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I don't see?
'before close of play big match is liable' is the wordplay.
I cannot quite see how this works, but
'match' could be 'test' (as in a test match in cricket) and 'test' is found within the answer.
'close of play' could be 'y' (closing letter of the word 'play') and 'y' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'to' is the link.
Can you help me to learn more?
(Other definitions for testy that I've seen before include "Captious" , "Irritably impatient" , "Irascible" , "Grumpy" , "Cross" .)