It's a measure of trade unionists lacking adroitness (4)
I believe the answer is:
unit
'it's a measure' is the definition.
(unit is a kind of measure)
'trade unionists lacking adroitness' is the wordplay.
'lacking' indicates named letters should be taken away.
'trade'+'unionists'='tradeunionists'
'tradeunionists' with 'adroitness' taken out is 'UNIT'.
'of' is the link.
(Other definitions for unit that I've seen before include "Some fighting men" , "Group that's part of a larger organisation" , "Single item, maybe part of a set" , "Specialised group" , "A single self-contained item" .)