A blackleg upset nursing union? You can count on it! (6)
I believe the answer is:
abacus
'you can count on it' is the definition.
(an abacus is a tool for doing sums)
'a blackleg upset nursing union?' is the wordplay.
'blackleg' becomes 'scab' (synonyms).
'upset' is a reversal indicator.
'nursing' means one lot of letters goes inside another.
'union?' becomes 'U' (common abbreviation as in 'Trades Union Congress').
'scab' back-to-front is 'bacs'.
'bacs' enclosing 'u' is 'bacus'.
'a'+'bacus'='ABACUS'
(Other definitions for abacus that I've seen before include "Early counting-frame" , "that one can count on" , "one counts on it" , "You could count on this once" , "summer once?" .)