Order half a case and you'll get fat! (5)
I believe the answer is:
obese
'fat' is the definition.
(similar in meaning)
'order half a case' is the wordplay.
'order' becomes 'OBE' (Order of the British Empire award).
'half a' suggests halving the letters.
'case' with half the letters taken is 'se'.
'obe'+'se'='OBESE'
'and you'll get' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for obese that I've seen before include "in need of slimming down" , "Too chubby" , "Dangerously corpulent" , "Enormous" , "carrying remarkable weight" .)