Plain caravan? I'll alter covers (7)
I believe the answer is:
vanilla
'plain' is the definition.
(I've seen this in another clue)
'caravan? i'll alter covers' is the wordplay.
'caravan?' becomes 'van' ('van' can be a synonym of 'caravan').
'covers' indicates taking the first letters (I've seen 'cover' mean this (in a down clue, the letter covering the word)).
The initial letter of 'alter' is 'a'.
'van'+'ill'+'a'='VANILLA'
(Other definitions for vanilla that I've seen before include "plant" , "lacking any special features" , "Flavouring derived from an orchid" , "without additions" , "devoid of special features" .)