Poser: one put on underwear, nearest pants (5-6)
I believe the answer is:
brain-teaser
'poser' is the definition.
(I've seen this before)
'one put on underwear nearest pants' is the wordplay.
'one' becomes 'i' (Roman numeral).
'put on' means one lot of letters go next to another (some letters go on or after others).
'underwear' becomes 'bra' (bra is a type of undergarment).
'pants' is an anagram indicator (UK informal term for 'nonsense' or 'rubbish').
'nearest' is an anagram of 'nteaser'.
'i' put after 'bra' is 'brai'.
'brai'+'nteaser'='BRAIN-TEASER'
(Other definitions for brain-teaser that I've seen before include "Riddle" , "Problem, puzzle" , "Poser" , "Puzzle requiring thought" , "Difficult puzzle" .)