Navy supports holding line (5)
I believe the answer is:
fleet
'navy' is the definition.
(I've seen this before)
'supports holding line' is the wordplay.
'supports' becomes 'feet' (foot is a kind of support).
'holding' is an insertion indicator.
'line' becomes 'l'.
'feet' going around 'l' is 'FLEET'.
(Other definitions for fleet that I've seen before include "Fast as a naval unit" , "Fast moving" , "Navy's ships" , "good turn of speed" , "bowler famously intercepted one once" .)