Gymnasts upset about source of unusual eye movements (9)

'eye movements' is the definition.
(nystagmus means movements of the eyes)

'gymnasts upset about source of unusual' is the wordplay.
'upset' indicates an anagram.
'about' means one lot of letters goes inside another.
'source of' suggests taking the first letters.
The first letter of 'unusual' is 'u'.
'gymnasts' with letters rearranged gives 'nystagms'.
'nystagms' placed around 'u' is 'NYSTAGMUS'.