The first feeble adjustment (5)
I believe the answer is:
tweak
'adjustment' is the definition.
(I have seen 'Fine adjustment ' mean 'tweak' so perhaps 'adjustment' could also mean 'tweak')
'the first feeble' is the wordplay.
'the first' becomes 't' (wst letter of 'the').
'feeble' becomes 'weak' (similar in meaning).
't'+'weak'='TWEAK'
(Other definitions for tweak that I've seen before include "Change slightly" , "Modify slightly" , "Make fine adjustment" , "Fine tune" , "Nip" .)