Bear in mind politician is under fire? Not half! (8)
I believe the answer is:
remember
'bear in mind' is the definition.
(I've seen this before)
'politician is under fire? not half' is the wordplay.
'politician' becomes 'member' (eg Member of Parliament).
'is under' says to put letters next to each other.
'not half' suggests halving the letters.
'fire' halved is 're'.
'member' after 're' is 'REMEMBER'.
(Other definitions for remember that I've seen before include "Recall to mind" , "Retain memory of" , "Be sure" , "Bear in mind" , "Call to mind" .)