Add one litre, tap turning (11)
I believe the answer is:
interpolate
'add' is the definition.
(thesaurus)
'one litre tap turning' is the wordplay.
'turning' is an anagram indicator.
'one'+'litre'+'tap'='onelitretap'
'onelitretap' with letters rearranged gives 'INTERPOLATE'.
(Other definitions for interpolate that I've seen before include "Insert (between other terms of a series)" , "Insert - not a reptile (anag)" , "put in the middle" , "go-between?" .)