Smear engineers in unusual task (6)
I believe the answer is:
streak
'smear' is the definition.
(I've seen this in another clue)
'engineers in unusual task' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'in' is an insertion indicator.
'unusual' indicates anagramming the letters.
'task' is an anagram of 'stak'.
're' put within 'stak' is 'STREAK'.
(Other definitions for streak that I've seen before include "Irregular mark - run naked" , "Run without clothes" , "Go faster" , "Spell (of good luck)" , "Move very fast; thin strip" .)