Occasionally call out that fellow as cover (6)
I believe the answer is:
clothe
'cover' is the definition.
('clothe' can be a synonym of 'cover')
'occasionally call out that fellow' is the wordplay.
'occasionally' indicates alternate letters.
'that fellow' becomes 'he' (I've seen this in other clues).
'call'+'out'='callout'
The alternating letters of 'callout' are 'clot'.
'clot'+'he'='CLOTHE'
'as' is the link.
(Other definitions for clothe that I've seen before include "Provide cover for body and limbs" , "Fit out with garments" , "Invest" , "Remote area in Mongolia/China" , "Put garments onto" .)