Clue: Tub with helm malfunctioning, mostly foreboding upper sides' curving (10)
Answer: TUMBLEHOME
Explanation: Definition: upper sides' curving
***(tub helm)** [anag:*malfunctioning*] + [*mostly*] **OME**(n) 
 
 
*Tumblehome is a shipbuilding term describing the inward curvature of the upper parts of the sides of some ships.