Old sects screening head of comet's eclipses (7)

'eclipses' is the definition.
(occult can mean to hide or eclipse)

'old sects screening head of comet's' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'sects' becomes 'cults' (cult is a type of religious sect).
'screening' indicates putting letters inside (inserted letters are covered or screened by others).
'head of' suggests taking the first letters.
The initial letter of 'comets' is 'c'.
'cults' going around 'c' is 'ccults'.
'o'+'ccults'='OCCULTS'