Show amusement apprehending one figure of speech (6)
I believe the answer is:
simile
'figure of speech' is the definition.
(simile is a kind of figure of speech)
'show amusement apprehending one' is the wordplay.
'show amusement' becomes 'smile'.
'apprehending' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'smile' placed around 'i' is 'SIMILE'.
(Other definitions for simile that I've seen before include "Figure of speech comparing one thing to another" , "Figure of speech expressing resemblance between different things" , "Figure of speech used in making comparisons" , "as easy as pie, perhaps" , "Figure of speech, eg, as cool as a cucumber" .)