Mother with trade union in fact (5)
I believe the answer is:
datum
'fact' is the definition.
(singular form of 'data')
'mother with trade union' is the wordplay.
'mother' becomes 'dam' (female parent of an animal).
'with' means one lot of letters goes inside another.
'trade union' becomes 'TU' (abbreviation for trade union).
'dam' placed around 'tu' is 'DATUM'.
'in' acts as a link.
(Other definitions for datum that I've seen before include "Given item" , "One item of information" , "Something given" , "Single informative fact" , "Single piece of information" .)