One putting on happy expression in comparison (6)
I believe the answer is:
simile
'comparison' is the definition.
(simile is a kind of comparison)
'one putting on happy expression' is the wordplay.
'one' becomes 'i' (Roman numeral).
'putting on' means one lot of letters goes inside another (some letters wear or 'put on' others).
'happy expression' becomes 'smile' (I've seen this before).
'i' inserted within 'smile' is 'SIMILE'.
'in' acts as a link.
(Other definitions for simile that I've seen before include "Figure of speech that expresses a resemblance between different things" , "as daft as a brush, say" , "as pleased as Punch perhaps?" , "Figure of speech in which one thing is likened to another" , "Figure of speech involving comparison" .)