Sign Keir's opponent is losing her head when drinking orange juice (5)
I believe the answer is:
emoji
'sign keir's opponent is losing' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'her head when drinking orange juice' is the wordplay.
'her' becomes 'Em'.
'head when' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'drinking' is an insertion indicator (inserted letters are consumed or drunk).
'orange juice' becomes 'oj' (abbreviation).
'em'+'i'='emi'
'emi' placed around 'oj' is 'EMOJI'.
Can you help me to learn more?
(Another definition for emoji that I've seen is " Graphic expression".)