Leading position a peer's had reallocated (9)
I believe the answer is:
spearhead
'leading position' is the definition.
(spearheading something is leading it)
'a peer's had reallocated' is the wordplay.
'reallocated' is an anagram indicator (letters reallocated to new positions).
'a'+'peers'+'had'='apeershad'
'apeershad' anagrammed gives 'SPEARHEAD'.
(Other definitions for spearhead that I've seen before include "initiate attack" , "front" , "those at the forefront" , "Part of weapon possibly iron" , "Vanguard" .)