The writer’s weapon to constrain junior academic bias (9)
I believe the answer is:
imbalance
'bias' is the definition.
(a bias is a lack of balance or equality)
'the writer's weapon to constrain junior academic' is the wordplay.
'the writer's' becomes 'I'm' (both could mean 'the crossword setter is').
'weapon' becomes 'lance' (lance is a kind of weapon).
'to constrain' indicates putting letters inside (constrain can mean to imprison).
'junior academic' becomes 'ba' (Bachelor of Arts).
'im'+'lance'='imlance'
'imlance' enclosing 'ba' is 'IMBALANCE'.
(Other definitions for imbalance that I've seen before include "Asymmetry" , "Disparity" , "disproportion" , "Partiality" , "Failure of equilibrium" .)