Sound reasonable, disguising a meekness (4,5)
I believe the answer is:
make sense
'sound reasonable' is the definition.
'disguising a meekness' is the wordplay.
'disguising' is an anagram indicator.
'a'+'meekness'='ameekness'
'ameekness' with letters rearranged gives 'MAKE SENSE'.
(Other definitions for make sense that I've seen before include "Slaughterhouse" , "Be intelligible" , "Add up" , "Be rational" , "Be understandable or practicable - it's logical" .)