When the referee's sick, replacement is called for (6)
I believe the answer is:
refill
'when the' is the definition.
I can't tell whether this defines the answer.
'referee's sick replacement is called for' is the wordplay.
I cannot really understand how this works, but
'sick' could be 'ill' (synonyms) and 'ill' is present in the answer.
'referee' could be 'ref' (synonyms) and 'ref' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for refill that I've seen before include "Replace contents" , "Solution for dried-up writer" , "Charge again" , "Top up glass, replenish" , "Top up glass or petrol tank" .)