Recalled expression used by sleepy outsiders, barely able to keep their eyes open (6)
I believe the answer is:
drowsy
'barely able to keep their eyes open' is the definition.
I can't judge whether this defines the answer.
'recalled expression used by sleepy outsiders' is the wordplay.
'recalled' says the letters should be written in reverse (to recall can mean to bring something back).
'expression' becomes 'word'.
'used by' means one lot of letters go next to another.
'outsiders' means to remove the middle letters (only the letters on the outside of the word remain).
'sleepy' with its centre taken out is 'sy'.
'word' back-to-front is 'drow'.
'drow'+'sy'='DROWSY'
Can you help me to learn more?
(Other definitions for drowsy that I've seen before include "Wanting to nod off" , "Almost asleep" , "Not being very alert" , "liable to drop off" , "Ready to sleep" .)