Reduce one in length, possibly (7)
I believe the answer is:
lighten
'reduce' is the definition.
(thesaurus)
'one in length possibly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'possibly' indicates an anagram.
'length' anagrammed gives 'lghten'.
'i' placed inside 'lghten' is 'LIGHTEN'.
(Other definitions for lighten that I've seen before include "Make less serious" , "Illuminate" , "Get blonde" , "Make brighter or less heavy" , "Make paler" .)