18 across
On
retreat,
say
love
of
the
earth
(3)
On retreat, say love of the earth (3)
hints
0
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The definition is at the end of the clue.
How the clue works
Look at of the earth — that's your definition, since geo- is the prefix meaning earth. For the wordplay, say gives you EG (as in "e.g."), which is then reversed by on retreat to give GE, and love chips in with O (zero in tennis), so you get GE plus O to round it all off.