Too apparent in allusion on occasion (4)
I believe the answer is:
also
'too' is the definition.
(synonyms)
'allusion on occasion' is the wordplay.
'on occasion' means one should take alternating letters.
The alternate letters of 'allusion' are 'ALSO'.
'apparent in' is the link.
(Other definitions for also that I've seen before include "revolutionary a part for Yugoslavia" , "Plus" , "an indication of more to come" , "Item" , "Old" .)