。
This descent is dangerous too — there's nothing to hold onto.
Literal
Here [genitive-の] descent [also-も] handhold [subject-が] exist-not [reason-ので] dangerous.
Mountain/hiking vocabulary. 下り ('descent,' from 下る 'to descend') and 手掛かり ('handhold, clue, foothold,' from 手 'hand' + 掛かり 'hanging') are both nouns derived from verbs. 手掛かり can mean both literal 'handhold' (in climbing) and metaphorical 'clue' (in investigations). Here it's literal, describing a dangerous descent with no secure grip.