。
Hold on to the handrail just in case.
Literal
By-chance's time [for-に備えて] handrail [at-に] grip-[command-なさい].
手すり ('handrail') — 手 ('hand') + すり (from 擦る, 'to rub,' here meaning a thing you grasp with your hand). つかまる ('to hold on, to cling') takes its target with に — the thing gripped is the goal of the clinging, not a direct object.