Over-ornate style will cause an endless row with two companies (6)
I believe the answer is:
rococo
'over-ornate style' is the definition.
I can't tell whether this definition defines the answer.
'an endless row with two companies' is the wordplay.
'two companies' means two replacements for 'company'.
'an endless' means to remove the last letter.
'with' is a charade indicator (letters next to each other).
'company' becomes 'co' (abbreviation).
'company' becomes 'co' (abbreviation).
'row' with its last letter taken away is 'ro'.
'ro'+'co'+'co'='ROCOCO'
'will cause' is the link.
Can you help me to learn more?
(Other definitions for rococo that I've seen before include "Eighteenth-century French architectural style" , "fine style" , "18th-century French style" , "Ornamental style associated with Louis XV" , "Grotesque" .)