Well done ragout contains head of lettuce chopped up (15)
I believe the answer is:
congratulations
'well done' is the definition.
(I've seen this before)
'ragout contains head of lettuce chopped up' is the wordplay.
'head of' suggests taking the first letters.
'chopped up' indicates anagramming the letters.
The initial letter of 'lettuce' is 'l'.
'ragout'+'contains'+'l'='ragoutcontainsl'
'ragoutcontainsl' is an anagram of 'CONGRATULATIONS'.
(Other definitions for congratulations that I've seen before include "well done" , "good show" , "Best wishes" , "Cliff Richard hit" , "praise" .)