Armstrong’s heard of what prayers do (5)
I believe the answer is:
kneel
'what prayers do' is the definition.
'armstrong's heard' is the wordplay.
'armstrong' becomes 'neil' (Neil Armstrong).
'heard' indicates a 'sounds like' (homophone) clue.
'neil' sounds like 'KNEEL'.
'of' is the link.
(Other definitions for kneel that I've seen before include "Genuflect" , "Rest in prayer" , "Adopt position for prayer or submission" , "Show obeisance" , "Assume humble posture" .)