About turn before head of infantry deserts - that's what they're good for (5)

I believe the answer is:

cacti

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'deserts that's what they're good for' is the definition.
The definition suggests an adverb but the answer is not.

'about turn before head of infantry' is the wordplay.
'about' becomes 'c' (circa).
'turn before' becomes 'act' (theatre. I am not sure about the 'before' bit.).
'head of' suggests taking the first letters.
The initial letter of 'infantry' is 'i'.
'c'+'act'+'i'='CACTI'

Can you help me to learn more?

(Other definitions for cacti that I've seen before include "Prickly plants" , "They store water" , "Plants of dry regions" , "Spiky desert plants in plural" , "Spiny succulents" .)

I've seen this clue in The Independent.

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