Gets thinner without one cooking beef up (10)
I believe the answer is:
strengthen
'beef up' is the definition.
(synonyms)
'gets thinner without one cooking' is the wordplay.
'without' is a deletion indicator.
'one' becomes 'i' (Roman numeral).
'cooking' indicates an anagram (cook can mean to falsify or distort).
'gets'+'thinner'='getsthinner'
'getsthinner' with 'i' taken away is 'getsthnner'.
'getsthnner' anagrammed gives 'STRENGTHEN'.
(Other definitions for strengthen that I've seen before include "Make tough" , "Toughen up" , "Fortify" , "Reinforce, substantiate" , "Bolster" .)