Fa-law kuntu waghlan fī al-rijāl la-ḍarranī is an imagined premise and its consequence: were he insignificant, other people's hostility would injure him. In the next verse, wa-lākin introduces his contrary claim of personal strength.
Grammatically, law introduces the conditional and la-ḍarranī supplies the answer, marked by an initial lām. The imagined weakness is a device for emphasizing the speaker's asserted independence.