。
The bike under the tree is mine.
Literal
Tree [possessive-の] under [at-に] exist bicycle [topic-は] my [possessive-の] is.
木の下にある ('which is under the tree') is a relative clause modifying 自転車 ('bicycle'). Built from the existence verb ある in plain form — a classic example of how Japanese can use a whole 'X is located at Y' clause as a noun-modifier. 私の ('mine,' literally 'my') is a possessive noun phrase, with the referent (bike) understood from context.