Shrubs in old beds have frail roots essentially (9)

'shrubs' is the definition.
(ocotillo is a kind of shrub)

'old beds have frail roots essentially' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'beds' becomes 'cots' (cot is a kind of bed).
'have' is an insertion indicator.
'frail' becomes 'ill' (I can't explain this - if you can you should believe this answer much more).
'essentially' means to look at the middle letters.
The middle letter of 'roots' is 'o'.
'ill'+'o'='illo'
'cots' enclosing 'illo' is 'cotillos'.
'o'+'cotillos'='OCOTILLOS'

'in' is the link.

Can you help me to learn more?