Reserve needed by Celtic eleven (3)
I believe the answer is:
ice
'reserve' is the definition.
(I've seen this before)
'needed by celtic eleven' is the wordplay.
'needed by' says the answer is hidden in the clue (the solution letters are needed to write the clue).
'ICE' can be found hidden inside 'celtic eleven'.
(Other definitions for ice that I've seen before include "Girl's best friends" , "A state of water" , "Cool down" , "Being feeble, can't cut any" , "Freeze" .)