Officers to lead Royal Artillery on steamship (3,5)
I believe the answer is:
top brass
'officers' is the definition.
(I have seen 'Senior officers ' mean 'top brass' so perhaps 'officers' could also mean 'top brass')
'to lead royal artillery on steamship' is the wordplay.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'royal artillery' becomes 'RA' (abbreviation).
'on' says to put letters next to each other.
'steamship' becomes 'ss' (nautical abbreviation).
'to'+'pb'+'ra'+'ss'='TOP-BRASS'
(Other definitions for top brass that I've seen before include "Authorities" , "Most important leaders, officers, etc" , "The people in charge (of the Army?)" , "Highest-ranking officials with best yellow alloy" , "high-ranking officials" .)