People concealing a purpose (4)
I believe the answer is:
mean
'purpose' is the definition.
(I've seen this before)
'people concealing a' is the wordplay.
'people' becomes 'men' (men can refer to humanity as a whole).
'concealing' means one lot of letters goes inside another.
'men' enclosing 'a' is 'MEAN'.
(Other definitions for mean that I've seen before include "middle course" , "Base; tight" , "Intend; unkind" , "Despicable" , "Intend; stingy" .)