Lead weapon used by chief (9)
I believe the answer is:
'lead' is the definition.
(spearheading is a kind of leading)
'weapon used by chief' is the wordplay.
'weapon' becomes 'spear' (spear is a kind of weapon).
'used by chief' becomes 'head' (I can't explain this - if you can you should believe this answer much more).
'spear'+'head'='SPEARHEAD'
Can you help me to learn more?
(Other definitions for spearhead that I've seen before include "the sort of point best missed" , "Take charge of" , "Vanguard" , "Advanced troops" , "front" .)