Lamented radical takeover of union (4)
I believe the answer is:
rued
'lamented' is the definition.
(I've seen this in another clue)
'radical takeover of union' is the wordplay.
I cannot quite understand how this works, but
'radical' could be 'r' (chemistry abbreviation) and 'r' is located in the answer.
'union' could be 'u' (common abbreviation as in Trades Union Congress) and 'u' is present in the answer.
The remaining letters 'ed' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rued that I've seen before include "'Regretted, repented (4)'" , "Was contrite about" , "Was sorry" , "Bitterly regretted" , "was remorseful" .)