It's a caper when one has only a half ticket (5)
I believe the answer is:
antic
'it's a caper' is the definition.
(synonyms)
'a half ticket' is the wordplay.
'a' becomes 'an'.
'half' indicates taking half.
'ticket' with half the letters taken is 'tic'.
'an'+'tic'='ANTIC'
'when one has only' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for antic that I've seen before include "Mad act done for fun" , "Caper; grotesque (arch.)" , "Prank" , "seen to be bizarre, at one time" , "Frolic" .)