Seated posture many adopting uncomfortably at first (5)
I believe the answer is:
lotus
'seated posture' is the definition.
(yoga posture)
'many adopting uncomfortably at first' is the wordplay.
'many' becomes 'lots' (I've seen this before).
'adopting' indicates putting letters inside.
'at first' indicates taking the first letters.
The first letter of 'uncomfortably' is 'u'.
'lots' going around 'u' is 'LOTUS'.
(Other definitions for lotus that I've seen before include "Plant widely regarded as sacred" , "Mythical plant inducing languor" , "Mythical fruit inducing dreamy state" , "Aquatic plant" , "Plant promoting forgetfulness" .)