Become tense when argument goes in several different directions (7)
I believe the answer is:
stiffen
'become tense' is the definition.
Although both the answer and definition are verbs in their base form, I cannot understand how they can define each other.
'argument goes in several different directions' is the wordplay.
'argument' becomes 'tiff' (I've seen this before).
'goes in' is an insertion indicator.
'several different directions' becomes 'sen' (I can't justify this - if you can you should give a lot more credence to this answer).
'tiff' inserted inside 'sen' is 'STIFFEN'.
'when' is the link.
Can you help me to learn more?
(Other definitions for stiffen that I've seen before include "Make hard" , "Become more rigid" , "Reinforce" , "Steel" , "Grow harder" .)