Stress caused by row with subordinate (9)
I believe the answer is:
underline
'stress' is the definition.
(underlining is a kind of stressing)
'row with subordinate' is the wordplay.
'row' becomes 'line' (I've seen this before).
'with' says to put letters next to each other.
'subordinate' becomes 'under' ('under' can be similar in meaning to 'subordinate').
'line' after 'under' is 'UNDERLINE'.
'caused by' acts as a link.
(Other definitions for underline that I've seen before include "Emphasize, stress importance" , "Method of showing emphasis in writing" , "Call attention to - highlight" , "Reinforce" , "Emphasise with horizontal stroke of pen" .)