Display staff one attached to iron post (13)
I believe the answer is:
manifestation
'display' is the definition.
(I've seen this in another clue)
'staff one attached to iron post' is the wordplay.
'staff' becomes 'man' (as in staffing or manning a particular place).
'one' becomes 'i' (Roman numeral).
'attached to' says to put letters next to each other.
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'post' becomes 'station' (synonyms).
'man'+'i'+'fe'+'station'='MANIFESTATION'
(Other definitions for manifestation that I've seen before include "Display" , "Appearance of a ghost" , "Sign" , "Symptom (of an ailment)" , "Demonstration" .)