US linesman adopting singular position (4)
I believe the answer is:
'position' is the definition.
(I know that pose is a type of position)
'us linesman adopting singular' is the wordplay.
'us linesman' becomes 'Poe' (Edgar Allan Poe - he wrote lines of poetry).
'adopting' means one lot of letters goes inside another (inserted letters are taken on or adopted).
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'poe' going around 's' is 'POSE'.
(Other definitions for pose that I've seen before include "Assume an attitude (for photo etc)" , "Sit for picture" , "Attitude struck" , "'Present, a problem say (4)'" , "Affectation" .)