Aspiring union leader in unusual low bed (5-2)
I believe the answer is:
would-be
'aspiring' is the definition.
(I've seen this before)
'union leader in unusual low bed' is the wordplay.
'leader' indicates taking the first letters.
'in' is an insertion indicator.
'unusual' indicates an anagram.
The initial letter of 'union' is 'u'.
'low'+'bed'='lowbed'
'lowbed' is an anagram of 'woldbe'.
'u' put into 'woldbe' is 'WOULD-BE'.
(Other definitions for would-be that I've seen before include "Pretender" , "Blew duo away for desiring or pretending" , "One aspiring to what they are not" , "See 13" .)