MacGregors' leader plunged into burn for cover (5)
I believe the answer is:
smear
'for cover' is the definition.
(smearing is a kind of covering)
'macgregors' leader plunged into burn' is the wordplay.
'leader' says to take the initial letters.
'plunged into' is an insertion indicator.
'burn' becomes 'sear' (sear can mean to burn).
The first letter of 'macgregors' is 'm'.
'm' placed within 'sear' is 'SMEAR'.
(Other definitions for smear that I've seen before include "Greasy or sticky mark" , "Slanderous denigration of character" , "Slur - may be studied microscopically" , "nasty mark" , "Blur, smudge" .)