Fail to impress when lured astray before Mass (10)
I believe the answer is:
underwhelm
'fail to impress' is the definition.
(I've seen this before)
'when lured astray before mass' is the wordplay.
'astray' is an anagram indicator.
'before' means one lot of letters go next to another.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'when'+'lured'='whenlured'
'whenlured' with letters rearranged gives 'underwhel'.
'underwhel'+'m'='UNDERWHELM'
(Other definitions for underwhelm that I've seen before include "Fail to impress" , "leave unmoved" , "Fail to excite" .)