Business discussed after hours as spa loth to change tack ultimately (8)
I believe the answer is:
shoptalk
'business discussed after hours' is the definition.
(shoptalk is talking about work in your free time)
'spa loth to change tack ultimately' is the wordplay.
'to change' is an anagram indicator.
'ultimately' says to take the final letters.
The final letter of 'tack' is 'k'.
'spa'+'loth'='spaloth'
'spaloth' anagrammed gives 'shoptal'.
'shoptal'+'k'='SHOPTALK'
'as' is the link.
(Another definition for shoptalk that I've seen is " Business discussion with others in the same business".)