One has to smile about the likeness (6)
I believe the answer is:
simile
'the likeness' is the definition.
(thesaurus)
'one has to smile' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
'to smile' could be 'smile' and 'smile' is found in the leftover letters.
No letters remain.
This explanation may well be incorrect...
'about' acts as a link.
Can you help me to learn more?
(Other definitions for simile that I've seen before include "as pleased as Punch perhaps?" , "Basic figure of speech, makes comparison" , "as silly as a loon, say" , "Figure of speech that expresses a resemblance between different things" , "Figure of speech comparing one thing to another" .)