The trouble when a hothead messes up sales (6)
I believe the answer is:
hassle
'the trouble' is the definition.
(synonyms)
'a hothead messes up sales' is the wordplay.
'a hothead' becomes 'h' (head letter of 'hot').
'messes up' is an anagram indicator (I've seen 'messed up' mean this).
'sales' anagrammed gives 'assle'.
'h'+'assle'='HASSLE'
'when' acts as a link.
(Other definitions for hassle that I've seen before include "Modern word for bother and annoyance" , "Annoy continually" , "An informal quarrel or upset" , "Difficulty - dispute" , "Nuisance, bother" .)